The word chorus can mean a lot of different things. Originally, the chorus was an ensemble of actors in Greek theatre who wore spooky masks and chanted or sang in unison (kind of like a choir). Chorus means pretty much the same thing in musical theatre today; you'll still hear actors say, “I tried out for a lead role, but all I got was a part in the chorus.” The chorus is there to reinforce the main actors by singing (and sometimes dancing in a “chorus line”) along with certain parts of a play to make them sound bigger and stand out — just like the chorus of a modern song.

At times, the term “refrain” has been used interchangeably with “chorus.” Technically, the refrain may be considered to be anything that's not the verse. In times past, the verse section was used to set up a story and introduce the song. After the verse alerted people that a song was about to begin, and filled in any “inside” information that the listener might need to understand the song, then the refrain came. The refrain functioned as the main body of the song and could contain several different versions of a chorus along with a bridge and instrumental sections. After the refrain kicked the song into high gear, it never went back to the verse. In the early decades of recorded music, the verse was often left out of a recording due to space considerations in available recording media (many early systems had less than three minutes of recording time) and the refrain was the only part that made it onto the record.

These days, a song part that contains the hook or title and appears more than once in a song is usually called “a chorus.” The chorus is still typically the biggest-sounding part of the song and the part most often accompanied by several voices. Fortunately, with the notable exception of a few heavy metal bands, back-up singers are no longer expected to wear spooky masks.