Function
A named recipe of code you write once and call whenever you need it.
A function wraps a few lines of code, gives them a name, and lets you run them on demand: define double(number) once, call it a thousand times. Functions can take inputs (parameters) and hand back a result (return).
They are the main tool programmers use against complexity: big problems get chopped into small named recipes, each easy to understand, test, and reuse.
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