Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiation of certain frequencies while passing others. Think sieve: pass what you want, reject all else.
For audio use the most common electronic filter is a bandpass filter, characterized by three parameters: centre frequency, amplitude (or magnitude), and bandwidth. Bandpass filters form the heart of audio graphic equalizers and parametric equalizers.
A device to remove dirt and other impurities from air, oil, or water.
A device used to reject certain frequencies of light.
See also: Adsorbent Filter, Anti-Aliasing Filter, Band Stop Filter, Bandpass Filter, Constant Percentage Bandwidth Filter, Convolution, Electronic Filter, Filtering, Full Flow Filter, Infinite Impulse Response Filter, Kalman Filter, Mechanical Filter, Optical Filter, Recursive Filter, Roll Off, Spatial Filtering, Strainer, Surface Filtration, Tracking Filter.