📖 Glossary
Every programming term from this course, defined in plain English with examples in all three languages. Use Ctrl+F / Cmd+F to search.
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A – D
API (Application Programming Interface)
A way for programs to talk to each other over the internet. Your program sends a request to a server and gets back data (usually JSON). Example: fetching weather data from wttr.in.
Lesson: 8.3 — Making API Requests
Argument
The actual value you pass to a function when you call it. Often confused with parameter — the parameter is the placeholder in the function definition; the argument is the real value.
greet("Alice") — "Alice" is the argument. In def greet(name), name is the parameter.
Array / List
An ordered collection of values, accessed by index (starting at 0). Python calls them lists, JavaScript and C# call them arrays (C# also has List<T>).
Python: [1, 2, 3] · JS: [1, 2, 3] · C#: new List<int> {1, 2, 3}
Lesson: 5.1 — Arrays and Lists
Async / Await
A pattern for writing code that waits for slow operations (API calls, file reads) without freezing the program. The function is marked async, and you await the slow operation.
Python: async def / await · JS: async function / await · C#: async Task / await
Lesson: 8.3 — Making API Requests
Boolean
A data type with only two values: true or false. Used in conditions and comparisons.
Python: True / False (capitalized!) · JS: true / false · C#: true / false
Lesson: 2.2 — Data Types
Class
A blueprint for creating objects. Defines what properties (data) and methods (behavior) an object will have. Like a cookie cutter — the class is the cutter, each object is a cookie.
Lesson: 6.1 — Classes and Objects
Closure
A function that "remembers" variables from the scope where it was created, even after that scope has ended. All three languages support closures.
Lesson: 4.3 — Scope and Closures
Concatenation
Joining strings together with the + operator. "Hello" + " " + "World" → "Hello World". String interpolation (f-strings, template literals, $-strings) is usually preferred.
Lesson: 2.2 — Data Types
Conditional
Code that runs only when a condition is true. Uses if, else if/elif, and else keywords.
Lesson: 3.1 — Conditionals
Constant
A variable whose value should never change after it's set. Python uses ALL_CAPS by convention. JS uses const. C# uses const or readonly.
Lesson: 2.1 — Variables and Constants
Constructor
A special method that runs when you create a new object from a class. It sets up the object's initial state.
Python: __init__(self) · JS: constructor() · C#: method with same name as the class
Dictionary / Object / Map
A collection of key-value pairs. Look up values by their key (like a real dictionary: look up a word to find its definition).
Python: dict · JS: plain object {} or Map · C#: Dictionary<K,V>
E – I
Exception / Error
A problem that occurs while the program is running (not a syntax error — the code is valid, but something went wrong). Handled with try/catch/except.
Lesson: 7.1 — Try, Catch, and Finally
Expression
Any piece of code that produces a value. 2 + 3, "hi".upper(), x > 5 are all expressions. A statement does something (like print()); an expression is something.
Float / Double
A number with a decimal point. Python: float · JS: number (all numbers are floats) · C#: double or float.
Lesson: 2.2 — Data Types
Function / Method
A reusable block of code that takes input (parameters), does something, and optionally returns output. A method is a function that belongs to a class or object.
Python: def greet(): · JS: function greet() · C#: static void Greet()
HTTP (HyperText Transfer Protocol)
The protocol used by the web. Your program sends an HTTP request (GET, POST, etc.) to a server and gets back an HTTP response with a status code and data.
Lesson: 8.3 — Making API Requests
Immutable
A value that can't be changed after it's created. Strings are immutable in all three languages — "hello".upper() creates a new string; it doesn't modify the original.
Index
The position of an item in a list/array, starting at 0. In ["a", "b", "c"], "a" is at index 0, "b" at index 1, "c" at index 2.
Lesson: 5.1 — Arrays and Lists
Inheritance
A class that "inherits" properties and methods from a parent class. The child class gets everything the parent has, plus can add or override behavior.
Python: class Dog(Animal): · JS: class Dog extends Animal · C#: class Dog : Animal
Integer
A whole number without a decimal point. 42, -7, 0 are integers.
Python: int · JS: number · C#: int
Lesson: 2.2 — Data Types
Iteration
Going through items in a collection one by one. A for loop iterates over a list. Something you can iterate over is called an iterable.
Lesson: 5.3 — Iterating Over Collections
J – O
JSON (JavaScript Object Notation)
A text format for storing and exchanging data. Looks like JavaScript objects but is language-independent. Most APIs return JSON. Keys must be quoted strings.
{"name": "Alice", "age": 30, "hobbies": ["reading", "coding"]}
Lesson: 8.2 — Working with JSON
Loop
Code that repeats. A for loop runs a set number of times. A while loop runs until a condition becomes false.
Lesson: 3.2 — Loops: for and while
Method
A function that belongs to an object or class. "hello".upper() — upper() is a method of the string object. See also: Function.
Mutable
A value that can be changed after creation. Lists/arrays are mutable — you can add, remove, and modify items. Contrast with immutable.
Null / None / Undefined
Represents "no value" or "nothing." Python: None. JS: null (intentional absence) and undefined (never assigned). C#: null.
Lesson: 2.1 — Variables and Constants
Object
An instance of a class. If Dog is the class (blueprint), then rex = Dog("Rex") creates an object (one specific dog). Also used loosely in JS to mean a key-value collection ({name: "Alice"}).
Lesson: 6.1 — Classes and Objects
Operator
A symbol that performs an operation. Arithmetic: + - * /. Comparison: == != < >. Logical: and/&& or/|| not/!.
Override
When a child class replaces a method from its parent class with its own version.
Python: just redefine it · JS: just redefine it · C#: requires virtual on parent and override on child
P – S
Parameter
A placeholder variable in a function definition. When you call the function, you pass an argument that fills in the parameter.
def greet(name): — name is the parameter. greet("Alice") — "Alice" is the argument.
Polymorphism
"Many forms" — the ability to call the same method on different types and get type-specific behavior. If Dog and Cat both have a speak() method, calling animal.speak() does the right thing regardless of which type animal is.
Property
A piece of data stored in an object. dog.name — name is a property of the dog object. Also called an attribute (Python) or field (C#).
Pseudocode
A plain-English description of a program's logic. Looks like code but uses human language. Helps you think through logic before writing real code.
Refactoring
Improving the structure of code without changing what it does. Examples: renaming variables, extracting helper functions, replacing deep nesting with early returns.
Lesson: 9.3 — Code Review and Next Steps
Return Value
The value a function sends back to the code that called it. Uses the return keyword. A function without a return returns None (Python), undefined (JS), or is void (C#).
Scope
Where a variable is visible and accessible. A variable declared inside a function is "local" to that function — it doesn't exist outside it. "Global" variables are accessible everywhere.
Lesson: 4.3 — Scope and Closures
Serialization
Converting an object into a format that can be saved (like JSON) or sent over a network. Deserialization is the reverse — creating an object from saved data.
Lesson: 8.2 — Working with JSON
Statement
A line of code that does something (assigns a variable, calls a function, loops). Contrast with an expression, which produces a value.
String
A sequence of text characters. Created with quotes: "Hello" or 'Hello'. Strings are immutable in all three languages.
Lesson: 2.2 — Data Types
String Interpolation
Embedding variables directly inside a string instead of concatenating with +.
Python: f"Hello, {name}" · JS: `Hello, ${name}` · C#: $"Hello, {name}"
Lesson: 2.2 — Data Types
T – Z
Truthy / Falsy
Values that act like true or false in a condition, even though they aren't boolean. Falsy values include: 0, "", None/null/undefined, empty collections. Everything else is truthy.
Try / Catch / Except
The error-handling structure. Code in the try block runs normally. If an error occurs, execution jumps to the catch/except block instead of crashing.
Python: try: / except: · JS: try {} catch {} · C#: try {} catch {}
Lesson: 7.1 — Try, Catch, and Finally
Type Coercion
When a language automatically converts one type to another. JavaScript does this aggressively ("5" == 5 is true!). Python and C# are stricter and usually require explicit conversion.
Type Conversion (Casting)
Explicitly converting a value from one type to another.
Python: int("42"), str(42) · JS: parseInt("42"), String(42) · C#: int.Parse("42"), 42.ToString()
Variable
A named container that stores a value. You can read it, change it, and pass it to functions.
Python: name = "Alice" · JS: let name = "Alice"; · C#: string name = "Alice";
Lesson: 2.1 — Variables and Constants
Version Control (Git)
A system that tracks every change to your code. Lets you undo mistakes, experiment safely, and collaborate with others. Git is the most popular version control system.
Lesson: 9.3 — Code Review and Next Steps