Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions. It emphasizes the use of pure functions, which produce the same output for the same input without side effects, making code easier to understand and test. This approach encourages immutability, meaning that data cannot be changed after it is created, leading to more predictable and reliable code. In functional programming, functions are first-class citizens, allowing them to be passed as arguments, returned from other functions, and assigned to variables. This flexibility enables developers to create higher-order functions, which can operate on other functions, enhancing code reusability and modularity. Popular languages that support this paradigm include Haskell, Scala, and JavaScript.