a mapping of a function, as a signal, that is defined in one domain, as space or time, into another domain, as wavelength or frequency, where the function is represented in terms of sines and cosines.

British Dictionary definitions for fourier transform

Fourier transform

noun

an integral transform, used in many branches of science, of the form F(x) = [1/√(2π)]ʃe i xy f(y)d y, where the limits of integration are from –∞ to +∞ and the function F is the transform of the function f