Merriam-Webster Collegiate Dictionary - fourier transform

 noun  Date: 1923 any of various functions (as F(u)) that under suitable conditions can be obtained from given functions (as f(x)) by multiplying by eiux and integrating over all values of x and that in scientific instrumentation describe the dependence of the average of a series of measurements (as of a spectrum) on a quantity of interest (as brightness) especially of a very small magnitude — called also ~ation