Chorus (effect)
An effect that makes one instrument sound like several by mixing in a copy with a wobbling delay.
When several players double a part they are never perfectly in time or in tune. Chorus fakes that: the signal is delayed by 15–35 ms, the delay is swept by an LFO so the pitch drifts slightly, and the copy is mixed back with the original. Shorter delays plus feedback give you a flanger instead, which is audibly the sweeping comb filter it is.
Roland built the effect into the JC-120 guitar amp in 1975 and sold it separately as the Boss CE-1 pedal a year later. It then became a signature of the 1980s, from clean guitar chords to the Juno-60's famous built-in chorus, which made a single oscillator sound wide. In stereo the two channels get different modulation and the image widens — though summed to mono the effect can partly cancel itself out.