Functional programming is a paradigm of programming that treats computation as the evaluation of mathematical functions and avoids changing state and mutable data. In this style, program logic is formulated using pure functions, which depend only on their input arguments and produce no observable side effects. These functions promote code predictability and ease of reasoning, as the output for a given set of inputs will always be the same, regardless of when or where the function is called. Concepts central to this paradigm include immutability, where data structures cannot be modified after creation, and higher-order functions, which allow functions to be treated as values that can be passed as arguments or returned from other functions. This mathematical foundation makes functional programming highly effective for concurrent and parallel computation, as the lack of shared, mutable state significantly reduces the potential for race conditions and complex debugging issues.