functional

The term functional is used across disciplines to describe something that serves a purpose within a system. In general speech it means relating to function, but in technical contexts it denotes a specific kind of object or concept defined by its role.

In mathematics, a functional is a map that takes a function as input and returns a scalar. Linear functionals map vectors to scalars linearly. Functionals are central in functional analysis, where the collection of continuous linear functionals forms the dual space; common examples include evaluation at a point and integration against a kernel.

In computer science, functional programming is a paradigm focused on pure functions, immutability, and first-class functions. Programs are built by composing and applying functions, avoiding side effects, which can aid reasoning and parallelism but may require different design approaches than imperative programming.

In nutrition, functional foods are those that provide health benefits beyond basic nutrition due to added nutrients or bioactive compounds. Definitions vary by jurisdiction; examples include fortified foods and probiotic products.

The adjective also appears in linguistics and engineering to describe roles, purposes, or structures that serve a function. Context determines whether functional refers to mathematical mappings, programming style, dietary effects, or organizational roles.