What is a parameter in programming?

In programming, a parameter refers to a function input that is defined during a function's declaration. When a function is created, parameters allow the function to accept values or arguments when it is called, enabling it to operate with different inputs. This is crucial as it makes functions reusable and flexible, allowing them to perform operations on varying data without needing to rewrite the code.

For instance, consider a function designed to calculate the area of a rectangle; it might have two parameters: length and width. When the function is called, different values for length and width can be supplied each time, allowing the same function to compute areas for different rectangles.

This understanding differentiates parameters from other programming concepts like return values, which relate to what the function produces after execution, or the notion of variables defined within the scope of the function, which are used internally. Thus, comprehending parameters is foundational for writing adaptable and efficient code.