An effect which adds a sense of spaciousness and "fullness" to a sound, as if it were being performed by a number of identical instruments playing in unison. It is a particularly useful effect on synthesizer outputs; many of the early polyphonic synths of the 1975-1985 time period included built-in chorus circuits (which helped improve the somewhat weedy sound of some of these synths). On bass sounds, chorusing dramatically alters the timbre, creating an effect as if the same sound were being played in two or three octave intervals above the input sound.
Roland made a name for itself with its chorus circuits in the early '80s, including them in its Juno series, its string synthesizers, a series of effect units, and even its Jazz Chorus guitar amplifers. (Today, the Jazz Chorus is one of the few solid-state guitar amps that guitarists regard as a classic.) The company's Dimension D chorus is renown for its subtlety and its ability to add space without sounding like an obvious effect.
The basic chorus effect is produced using a delay line in the range of 8 - 20 milliseconds, with some modulation of the delay time. See also flanging and doubling.