What is the meaning of 'chorus' in terms of a 12 bar blues?

Question

For example, its use in the phrase:

to play a total of four choruses on an A minor blues progression

Answer 1

It means once through the 12 bars. "Chorus" typically means once through the form that you solo on in any improvisational context like blues or jazz. So if the form were different and lasted 32 bars, then it would mean one iteration through that 32-bar form. In this case it's 12 bars.

Answer 2

A lot of pop songs from 80 or so years ago would have a pre-amble to the song itself - often seemingly bearing no relationship to the rest of the song - which is the part most people will know best. In fact,the verse for most songs seems to have been lost in the mists of time.

The beginning was labelled 'verse', and the main body 'chorus', although terms such as 'middle eight' and 'bridge' also get used for some parts of songs.

In a blues, and especially jazz situation, where a whole main part is played over and over, usually giving soloists opportunity to improvise over the chord sequence, that section is known as the 'chorus'. In a traditional 12-bar it's 12 bars long, and usually has a turn-around chord at the end of each chorus.

As already stated, a 'chorus' could be 8 bars, 16, or 32 bars long, but when it reaches the end, that chorus is over - it's someone else's turn to solo over the next n bars of 'chorus'.

Answer 3

I'm just adding score examples to @Tim's answer.

In musicals the numbers often have a kind of introductory first part leading to the main song. The two parts are often called the "verse" and "chorus", although they aren't always labeled as such in scores. Don't confuse this usage with "verse" and "chorus" as used in rock music. They are very different in terms of form.

Some jazz songs like I Got Rhythm use only the "chorus" part of the original musical number. That particular chorus is 32 bars. The band plays the chords while a soloist improvises. The 32 bar form will be repeated for a long solo or for other plays to take their turn. Each 32 bars played is called "taking or chorus" or something like that.

12 bar blues obviously doesn't have the introductory "verse" like the numbers in a musical, but the lingo of "taking a chorus" is applied to simply mean "play over the chord changes."

An example with the "chorus" actually labeled...

source: https://www.loc.gov/item/ihas.100004622/

I Got Rhythm is a more famous example, but in this score the first part is not labeled "verse" and the second part is labeled "refrain" rather than "chorus."