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A named slot in a function definition where the caller's value gets plugged in.
A local variable declared in a function's signature that receives the value the caller passes. The value passed in is called an 'argument'. Parameters can have defaults, rest spreads, and destructuring patterns.
Distinguished from arguments: the parameter is the slot in the declaration; the argument is the value at the call site. Pass-by-value copies primitives; pass-by-reference (or pass-by-sharing in JS) shares the same object instance, so mutations are visible to the caller.