What defines a function in Python?

A function in Python is defined as a block of reusable code that performs a specific task. This encapsulation allows for modularity and reusability in programming. Functions can take inputs, process them, and return outputs without altering the main program's structure. This makes code more organized and manageable, as complex tasks can be broken down into smaller, simpler functions that can be called multiple times throughout a program.

When you define a function, you establish a clear purpose for that block of code, which can make it easier to maintain and debug. By using functions, you can avoid redundancy since the same piece of code does not need to be written multiple times for similar tasks. This design principle is fundamental to both functional programming and modular code design in Python and other programming languages.