A function definition is a formal way to describe a specific task or operation that a program can perform. It typically includes a name, a list of parameters (inputs), and a block of code that executes when the function is called. Functions help organize code, making it reusable and easier to read. In programming languages like Python or Java, defining a function allows developers to encapsulate logic and perform complex calculations or actions with simple commands. This modular approach enhances code maintainability and reduces redundancy, as the same function can be invoked multiple times throughout a program.