Filter may refer to:
In chemistry, engineering, or household usage, a device to separate mixtures. See:
In optics and photography, a device to remove, or to enhance, certain ranges of wavelengths (colors) of light. See:
In signal-processing applications (such as telecommunications, music, and picture processing): a mathematical algorithm, or an electrical or mechanical device, to modify the contents of signals and to extract their salient features. See:
See filtering problem (stochastic processes) for the mathematical formalism.