Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids changing state or mutable data. It emphasizes the use of functions as the primary building blocks of software, promoting code that is easier to understand and maintain. In functional programming, functions are first-class citizens, meaning they can be passed as arguments, returned from other functions, and assigned to variables. This approach leads to more predictable and reliable code, as it minimizes side effects and encourages immutability, making it a popular choice for modern software development.