Functional programming (FP) is a programming paradigm centered around the use of pure functions and immutability, treating computation as the evaluation of mathematical functions without changing the state or data outside of the function. 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 emphasizes the use of functions that produce the same output for the same input, with no side effects, which leads to more predictable and maintainable code.

Functional programming languages like Haskell, Lisp, and Scala are designed to support this paradigm, but FP concepts can also be applied in languages like JavaScript, Python, and Java. Key features of functional programming include higher-order functions, recursion, and techniques such as map, filter, and reduce for processing collections of data. By minimizing side effects and relying on immutable data, functional programming makes it easier to reason about code, debug, and test, particularly in complex or concurrent systems. As software development continues to evolve, functional programming principles are increasingly being adopted even in traditionally imperative or object-oriented languages.