A device that is capable of removing or reducing a select band of frequencies within a signal.

An electronic circuit designed to attenuate a sound source's energy at a particular frequency. A true filter is always passive, in that it does nothing to add to the signal. However, most filters these days have amplifiers attached to them to allow the user to both boost and cut particular frequencies (active filters.) Multiple filters can be combined to create equalizers that are either passive, active or both.
(See Equalizer)