Equalizer (EQ)
A bank of filters that boosts or cuts chosen frequency ranges — the most used tool in mixing, and the most overused.
Each band has three controls: frequency, gain and Q, which sets how narrow the band is. A graphic EQ has fixed bands with one slider each, familiar from every 1980s hi-fi rack; a parametric EQ lets you move and narrow the bands at will. George Massenburg described the parametric design in an AES paper in 1972, and today it ships in every DAW. Shelving filters lift or cut everything above or below a corner frequency; high-pass filters remove the rumble outright.
Conventional EQs shift the phase around whatever they touch. Linear-phase EQs avoid that with long FIR filters, at the cost of real latency that the host has to absorb through delay compensation. Engineers like to say you should cut rather than boost — advice ignored about as often as it is given.