Chorus Vs. Choir 

The terms "chorus" and "choir" or most often interchangeable. They both commonly refer to an ensemble of singers that perform.

If you aren't sure which term to use for your group of singers, no matter how big or small, it really makes no difference, and it is up to you to see what term sounds better with the name of your group.

For example, you can have a group of singers be a chamber choir or a chamber chorus. 

The terms choir and chorus both originate from the music of ancient Greece in the 2nd century BC.

The original Greek chorus or choirs would sing their part in the overall works of Greek dramas. These first choirs would often sing or chant in unison without having any harmony parts.

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When Religious Settings Change The Meaning 

While this is not a definite rule, many people assume that religious groups gathering to sing are more often referred to as choirs rather than choruses.

Churches have choir stands where singers perform, and the chorus is more often referred to in a theatre or concert setting that involves secular music.

Regardless, I have seen church groups refer to themselves as choruses, so this is not a rule that is set in stone for sure. 

When The Type Of Performers Can Change The Meaning

Some dictionaries, including the Collins dictionary, refer to a chorus as a group of singers that can also include dancers and instrumentalists. Going back to the history of Greek choruses, in the theatrical culture, singing and dancing were often grouped together.

The word choros or "khoros" actually translates to dance in the Greek language.

People also like to call a larger group of singers a chorus, while they call a smaller group of singers a choir. Again, none of these rules are set in stone, so these terms can still be interchangeable. 

When "Chorus" Refers To A Song Section

A chorus in music is also a section of a song that is repeated after the verses.

This is also referred to as a refrain in a song, but the terms "refrain" and "chorus" can also have separate meanings due to some technicalities that you can read more about here. 

The chorus of a song is often the part that stands out and has layered vocals, which is a reference to the parts of a song where a chorus of people will come in to highlight and repeat the most emphasized section.

The chorus of a song also often has harmonies and more instrumentation than the verse sections of a song. 

Related Post: Tips For Layering Your Vocals Like A Choir