(

Mathematics:)

A

*functional* is a

function from a

space of functions to

scalars. Usually

linear functionals are intended, and the prefix "

linear" may even be dropped.

An example of a linear functional on the space **C**_{0}(R) of continuous functions on **R** that tend to 0 is φ(f)=f(0) ("substitution"). An example of a nonlinear functional is the variation of f.