1. A Fourier spectral representation of physical space. By computing the Fourier transform of a physical signal, such as temperature measurements frequently sampled during some period, the resulting amplitudes of the sine and cosine waves are an alternative way of describing the data. These amplitudes represent the signal in phase space. 2. A technique used in nonlinear dynamics and chaos theory to examine processes by the evolving relationship between dependent variables, rather than by the relationship between dependent and independent variables. See fast Fourier transform (FFT).