Hardware device or computer plugin used to alter the frequency balance of an audio source. An equalizer has the ability to boost or cut specific frequency ranges based around a center frequency and can be used either creatively, to drastically alter a source, or in a corrective manner, such as in a mastering situation, to even out anomalies in a mix. Equalizers come in various forms, including parametric and graphic, active or passive.
(See Filter, Graphic, Parametric)