Glossary
139 terms, explained without jargon.
139 terms
CPU
How computers workThe general-purpose thinking part of a computer that follows instructions in order.
Example: A slow report that does heavy calculations is usually waiting on the CPU.
GPU
How computers workA chip that does thousands of similar calculations at once, used for graphics and AI.
Example: Video editing and 3D games lean heavily on the GPU.
RAM
How computers workFast short-term memory that holds whatever the machine is working on right now.
Example: Many open browser tabs fill RAM and slow the laptop down.
Storage
How computers workLong-term space (SSD or hard disk) that keeps files after the machine is switched off.
Example: Your photos survive a restart because they live in storage, not RAM.
Operating system
How computers workThe software that shares the hardware between apps and enforces permissions.
Example: Windows, macOS, Linux, Android and iOS are operating systems.
Process
How computers workA running program with its own slice of memory.
Example: Each browser tab often runs as its own process, so one crash does not kill the rest.
File
How computers workA named block of stored bytes, organised in folders.
Example: invoice.pdf is a file; the folder it sits in is just a label for grouping.
Variable
Programming basicsA named box that holds a value while the program runs.
Example: cartTotal = 1200 stores the number 1200 under the name cartTotal.
Type
Programming basicsWhat kind of value something is: text, number, true/false, list and so on.
Example: The text "5" and the number 5 look alike but behave differently.
Boolean
Programming basicsA value that is either true or false.
Example: isPaid = true records that an order has been paid.
Condition
Programming basicsA yes/no check that decides which path the code takes.
Example: If the total is above ₹999, shipping is free.
Loop
Programming basicsRepeating the same work for each item in a collection.
Example: Calculating tax once per line item on an invoice.
Function
Programming basicsA named, reusable recipe that takes inputs and returns a result.
Example: calculateTax(amount) can be used from every screen that shows a price.
Array
Programming basicsAn ordered list of values.
Example: [100, 250, 400] is an array of three prices.
Object
Programming basicsA group of named values kept together.
Example: { name: "Asha", city: "Pune" } keeps related details in one place.
State
Programming basicsWhat is true in the app right now: who is signed in, what is in the cart.
Example: Two screens disagreeing about the cart count is a state bug.
Language
The technology mapThe syntax and rules you write instructions in.
Example: JavaScript, Python, Swift, Kotlin, Go and Rust are all languages.
Library
The technology mapReusable code you call to do one job.
Example: A date library formats '3 days ago' so you never write that logic.
Framework
The technology mapA structure for the whole app that calls your code at the right moments.
Example: You call a library; a framework calls you.
Runtime
The technology mapThe environment that actually executes your code.
Example: Browsers and Node.js are two different runtimes for JavaScript.
Compiler
The technology mapA tool that translates source code into machine code before it runs.
Example: Go and Rust are compiled, which catches some mistakes before shipping.
Package manager
The technology mapA tool that downloads and tracks third-party code.
Example: npm for JavaScript, pip for Python, Cargo for Rust.
Lock file
The technology mapA record of the exact dependency versions used, so every machine builds the same.
Example: package-lock.json prevents 'works on my machine' surprises.
IDE
The technology mapAn editor with built-in tools: error highlighting, navigation and debugging.
Example: VS Code, Xcode and Android Studio are common choices.
URL
How the web worksA web address made of a scheme, domain, path and optional query.
Example: https://shop.example.com/products/42?sort=price
DNS
How the web worksThe system that turns a domain name into an IP address.
Example: Cached DNS answers are why a domain change can take hours to spread.
HTTP / HTTPS
How the web worksThe rules for sending requests and responses on the web; HTTPS is the encrypted form.
Example: The padlock in the address bar means HTTPS is in use.
Request
How the web worksA message asking a server to read, create, change or delete something.
Example: GET /api/orders/42 asks to read one order.
Response
How the web worksWhat the server sends back: a status code and usually some data.
Example: 201 Created means a new record was made.
Status code
How the web worksA number summarising how a request went: 2xx ok, 4xx your fault, 5xx server's fault.
Example: 403 means signed in but not allowed.
HTML
How the web worksThe structure and content of a web page.
Example: Headings, paragraphs, buttons and forms are HTML.
CSS
How the web worksThe presentation layer: colour, spacing, layout and responsiveness.
Example: Changing one CSS variable can restyle the whole app.
JavaScript
How the web worksThe language browsers run to add behaviour to pages.
Example: Filtering a list as you type is JavaScript at work.
Browser
How the web worksThe program that fetches web pages, renders them and runs their JavaScript.
Example: Chrome, Safari and Firefox each have their own rendering engine.
Component
FrontendA reusable piece of interface with its own markup, style and behaviour.
Example: One Button component used on every screen keeps the app consistent.
Routing
FrontendMapping a URL to a screen.
Example: /glossary shows the glossary page and can be shared as a link.
Responsive design
FrontendOne layout that adapts to any screen width instead of separate sites.
Example: Cards stack on a phone and sit side by side on a laptop.
Accessibility (a11y)
FrontendMaking an app usable with screen readers, keyboards and low vision.
Example: Labelled inputs and visible focus outlines are the basics.
Form validation
FrontendChecking user input and explaining clearly what needs fixing.
Example: 'Phone number needs 10 digits' beats 'Something went wrong'.
Local state
FrontendData that only one component needs.
Example: Whether this dropdown is open right now.
API
Backend and dataA contract describing which requests a service accepts and what it returns.
Example: The app asks the API instead of touching the database directly.
JSON
Backend and dataA plain-text format of keys and values used to exchange data.
Example: {"id": 42, "paid": true}
Database
Backend and dataOrganised, queryable storage for your application's data.
Example: Orders, customers and products each get a table.
SQL
Backend and dataThe language for asking questions of a relational database.
Example: SELECT city, COUNT(*) FROM orders GROUP BY city;
Relationship
Backend and dataA link between tables, usually by storing an id.
Example: An order stores customer_id instead of repeating the customer's name.
CRUD
Backend and dataCreate, Read, Update, Delete — the four basic data operations.
Example: Most admin screens are just CRUD over one table.
Object storage
Backend and dataCheap storage for large files, referenced from the database by link.
Example: Product videos and PDFs live here, not in a database column.
Authentication
Backend and dataProving who a user is.
Example: Signing in with an email and password.
Authorization
Backend and dataDeciding what a signed-in user is allowed to do.
Example: Only admins may issue refunds — checked on the server.
Native app
Mobile appsAn app written with a platform's own tools and languages.
Example: Swift for iOS, Kotlin for Android.
Cross-platform
Mobile appsOne codebase producing apps for several platforms.
Example: Flutter and React Native are two common options.
WebView
Mobile appsA browser window embedded inside an app shell.
Example: Capacitor wraps an existing web app for the app stores.
Code signing
Desktop appsProving an app came from you and has not been tampered with.
Example: Unsigned desktop apps trigger operating-system warnings.
Electron
Desktop appsA way to ship a web app as a desktop app by bundling a browser engine.
Example: Bigger download, faster reuse of an existing web codebase.
Game engine
GamesA toolkit providing rendering, physics, audio, input and an editor.
Example: Unity, Unreal and Godot are widely used engines.
Game loop
GamesThe cycle of reading input, updating the world and drawing a frame.
Example: At 60 fps the whole cycle must finish in about 16 milliseconds.
Git
Shipping softwareVersion control that records every change and lets you go back.
Example: Reverting a bad release takes minutes instead of hours.
Commit
Shipping softwareA saved point in history with a message describing the change.
Example: 'Fix tax rounding on invoices' is a useful commit message.
Branch
Shipping softwareA parallel line of work that does not disturb the main version.
Example: Build a feature on a branch, then merge it when ready.
CI/CD
Shipping softwareAutomated checks on every change, and automated deployment once they pass.
Example: Tests run, a preview deploys, then production updates.
Environment
Shipping softwareA separate copy of the system: development, staging or production.
Example: Staging has its own database so tests never touch real orders.
Secret
Quality and costsA key, password or token that must never reach the browser or Git.
Example: Keep API keys on the server and rotate them if they leak.
Backup vs rollback
Quality and costsA backup restores data; a rollback restores a previous version of the code.
Example: Lost orders need a backup — a code rollback will not bring them back.
Logs
Quality and costsThe record an app writes of what it did and what failed.
Example: Logs turn a production mystery into a five-minute read.
Acceptance criteria
Working with AI agentsThe observable result that proves a task is genuinely done.
Example: 'Submitting the form shows a confirmation and sends an email.'
Hashing
Security and safetyTurning text into a fixed scrambled value that cannot be turned back.
Example: Passwords are stored as hashes, so even the company cannot read them.
Authorisation
Security and safetyDeciding what an identified user is allowed to do.
Example: An admin can delete users; a customer cannot.
Two-factor authentication
Security and safetyA second proof beyond the password, such as a code or a phone prompt.
Example: Turn it on for anything that controls money or code.
HTTPS
Security and safetyAn encrypted connection between the browser and the server.
Example: The padlock means nobody on the café wifi can read the traffic.
Key rotation
Security and safetyReplacing a secret key with a new one and disabling the old.
Example: The only real fix after a key leaks.
SQL injection
Security and safetyAn attack where user input is treated as database instructions.
Example: Prevented by using query parameters instead of glued-together text.
XSS
Security and safetyAn attack where someone else's text runs as code in your page.
Example: Escaping output stops a comment from running a script.
Least privilege
Security and safetyGiving each person, service or agent only the access it truly needs.
Example: A reporting script gets read-only access, not admin.
Schema
Databases and dataThe shape of a database: tables, columns and how they relate.
Example: Adding a field means changing the schema.
Foreign key
Databases and dataA column that points at a record in another table.
Example: An order stores customer_id instead of copying the customer's details.
Index
Databases and dataA lookup structure that makes searching a column fast.
Example: An index on email turns a nine-second search into milliseconds.
Migration
Databases and dataA recorded, repeatable change to the database structure.
Example: Adding a column ships as a migration, in version control.
NoSQL
Databases and dataDatabases that store flexible documents instead of fixed tables.
Example: Good for loose, fast-changing data; weaker guarantees for money.
Transaction
Databases and dataA group of database changes that all succeed or all fail together.
Example: Money leaving one account and arriving in another must be one transaction.
REST
APIs and integrationsA common style of web API using HTTP methods and addresses.
Example: GET to read, POST to create, DELETE to remove.
Webhook
APIs and integrationsA call the other service makes to you when something happens.
Example: The payment provider tells your server the payment succeeded.
Idempotency
APIs and integrationsDoing the same operation twice has the same effect as doing it once.
Example: Stops a retried webhook from creating two orders.
Rate limit
APIs and integrationsA cap on how many requests you may send in a period.
Example: Exceeding it returns 429 until you slow down.
Pull request
Git and teamworkA proposed change, shown as a diff, reviewed before merging.
Example: One intention per pull request keeps review real.
Merge conflict
Git and teamworkTwo branches changed the same lines, so a human must choose.
Example: Routine, and rare when branches are short-lived.
Serverless
Cloud and infrastructureCode that runs on demand and bills per request, with no server to manage.
Example: Ideal for traffic that is quiet most of the day.
Container
Cloud and infrastructureAn app packaged with everything it needs, so it runs identically anywhere.
Example: Docker images end the 'works on my machine' argument.
CDN
Cloud and infrastructureA network of servers that keeps copies of your files near users.
Example: An image loads from Mumbai instead of Virginia.
Egress
Cloud and infrastructureData leaving your cloud provider, usually billed per gigabyte.
Example: The most common cause of a surprise cloud bill.
Latency
Performance and scaleThe delay before a response starts arriving.
Example: Distance and slow queries are the two usual sources.
N+1 query
Performance and scaleFetching a list, then running one extra query per item.
Example: Fifty orders become fifty-one database round trips.
Queue
Performance and scaleA waiting line of background jobs processed after the response.
Example: Signup returns instantly and the welcome email is sent later.
Core Web Vitals
Performance and scaleGoogle's measures of loading, responsiveness and visual stability.
Example: LCP, INP and CLS — each maps to a real frustration.
Token
AI, LLMs and agentsA word fragment; the unit a language model reads and writes.
Example: Cost and context limits are both counted in tokens.
Context window
AI, LLMs and agentsHow much text a model can consider at once.
Example: Long chats forget early instructions once it fills up.
Hallucination
AI, LLMs and agentsA fluent, confident answer that is simply not true.
Example: An invented library function that looks entirely plausible.
Embedding
AI, LLMs and agentsText converted to numbers that capture meaning, for search by idea.
Example: 'Get my money back' finds the refunds page.
RAG
AI, LLMs and agentsRetrieving your own documents and letting the model answer from them.
Example: The standard way to make an assistant know your product.
Fine-tuning
AI, LLMs and agentsFurther training a model on examples to change how it behaves.
Example: Teaches style and format, not current facts.
Local model
AI, LLMs and agentsA model that runs on your own machine instead of a provider's.
Example: Private and free to run, usually weaker than the best hosted models.
Agent
AI, LLMs and agentsA model given tools and a loop, so it can act and check results.
Example: Needs narrow permissions and a human on irreversible steps.
SSR
Modern web stackServer-side rendering: the server sends finished HTML.
Example: Fast first view and visible to search engines.
TypeScript
Modern web stackJavaScript with types, so mismatches are caught while writing.
Example: The cheapest safety net when an agent writes code.
Build tool
Modern web stackSoftware that turns source files into optimised files to ship.
Example: Vite also runs the dev server that refreshes as you type.
Unit test
Testing and debuggingA tiny automated check that runs one function on its own.
Example: A test that checks the discount function returns 90 for 100 minus 10%.
End-to-end test
Testing and debuggingAn automated test that drives the real app in a browser like a user would.
Example: A script that signs in, adds an item and checks out on every release.
Regression
Testing and debuggingSomething that used to work and broke again after a later change.
Example: The discount bug returning three months after it was fixed.
Test coverage
Testing and debuggingThe share of your code that tests actually run.
Example: 80% coverage with no test on checkout is worse than 30% with one.
Stack trace
Testing and debuggingThe list of function calls that led to an error.
Example: Reading it top-down shows the first line of your own code involved.
Reproduce
Testing and debuggingMaking a bug happen reliably on demand.
Example: Creating a new account to trigger the failure new users reported.
Bisect
Testing and debuggingHalving the search space repeatedly until you find the cause.
Example: git bisect finds the exact commit that introduced a bug.
Breakpoint
Testing and debuggingA marker that pauses running code so you can inspect it.
Example: Pausing in the browser devtools to see what a variable holds.
Observability
Watching a live appBeing able to tell what a running system is doing from the outside.
Example: Logs, metrics and traces together make an app observable.
Structured logging
Watching a live appWriting logs as searchable fields rather than sentences.
Example: Logging userId and duration as fields lets you filter and chart them.
Metric
Watching a live appA number tracked over time.
Example: Requests per minute, error rate, or signups per day.
Distributed trace
Watching a live appA record of one request's journey through every service it touched.
Example: A trace showing 4 of 4.3 seconds were spent in one database call.
Alert fatigue
Watching a live appIgnoring alerts because too many are false alarms.
Example: After twenty noisy pages, nobody checks the twenty-first.
Incident
Watching a live appAn unplanned event that degrades service for users.
Example: Checkout failing for eleven minutes after a deploy.
Postmortem
Watching a live appA short blameless write-up of what happened and what will change.
Example: The outage report that added a smoke test to the deploy pipeline.
Uptime
Watching a live appThe share of time a service is available.
Example: 99.9% uptime allows about 43 minutes of downtime a month.
Screen reader
Accessibility and good UXSoftware that reads a screen aloud for blind and low-vision users.
Example: It announces headings, links and buttons in page order.
Alt text
Accessibility and good UXA short description of an image for people who cannot see it.
Example: alt="Bar chart: sales doubled in June".
Colour contrast
Accessibility and good UXHow clearly text stands out from its background.
Example: Light grey on white fails; dark grey on white passes.
Focus ring
Accessibility and good UXThe visible outline showing which element the keyboard is on.
Example: Removing it makes an app unusable without a mouse.
Semantic HTML
Accessibility and good UXUsing elements for their meaning, not just their looks.
Example: button, nav and h1 instead of styled divs.
WCAG
Accessibility and good UXThe international accessibility guidelines websites are measured against.
Example: AA level is the common legal and practical target.
Empty state
Accessibility and good UXWhat a screen shows when there is no data yet.
Example: A new dashboard explaining what appears here and offering one action.
Skeleton screen
Accessibility and good UXGrey placeholder shapes shown while content loads.
Example: Grey bars where the list will appear, instead of a blank page.
Webhook signature
Shipping to real usersA signed header proving a webhook really came from the provider.
Example: Verifying the Stripe signature before granting access.
Idempotent
Shipping to real usersSafe to run more than once with the same result.
Example: Processing the same payment event twice upgrades the account once.
Merchant of record
Shipping to real usersThe company legally selling to your customer and handling taxes.
Example: Paddle acts as merchant of record; with Stripe, you usually are.
Dunning
Shipping to real usersRetrying and chasing failed subscription payments.
Example: Emails and retries after a card expires.
PCI DSS
Shipping to real usersThe security standard for handling card data.
Example: Using a hosted payment form keeps you out of most of its scope.
Feature flag
Shipping to real usersA switch that turns a feature on or off without deploying.
Example: Releasing a new checkout to 5% of users first.
Activation
Shipping to real usersA new user reaching the product's first real value.
Example: Finishing a first lesson, not just signing up.
Retention
Shipping to real usersHow many users come back after their first visit.
Example: Week-two retention is a better health signal than total signups.
Permissive licence
Shipping to real usersA licence letting you use code commercially with attribution.
Example: MIT, Apache 2.0 and BSD.
Copyleft licence
Shipping to real usersA licence requiring derived work to use the same licence.
Example: GPL, and AGPL which extends this to hosted services.
GDPR
Shipping to real usersEuropean rules on collecting and handling personal data.
Example: Consent for tracking, and deleting data on request.