A chorus is the AABA portion (or refrain) of a classic 32-bar song form.
Not all songs have choruses, but the ones that do are easily identified because a chorus contains the most memorable music and lyrics in the song. It is repeated after each verse and usually contains the hook. A chorus can be removed from the rest of the song and still sound complete in itself both musically and lyrically. An example is: "We all live in a yellow submarine..."
In jazz, a chorus any complete statement of a composition's repeating harmonic pattern. If the piece in question is a 32-bar song form, then to "take a chorus" would mean playing a solo over that 32-bar chorus; but the same term is also used for soloing over a set of changes that doesn't fit that form (e.g., the Gershwins' "But not for me," or Thelonious Monk's "Straight, no chaser").
The term 'chorus' may also refer to an electronic delay effect that modulates the audio signal to create interference effects (colloquially described as "thickening" the sound, and/or making it "shimmer"). Often, the delay time and level of the copy is continuously varied in the interests of added realism. This part of the process, known as flanging (see flange), is often used as an effect in itself. The difference between the sound of the original signal and the chorused version resembles that between the sound of the same tone as sung by one singer and as sung by an entire choir or chorus of singers.
A chorus is also a group of singers. See choir.
A chorus, too,is a work written for a group of singers that can also involve instrumentalists.