MORE INFO ON NND CHORUSES.

Choruses on Nico Nico Douga began as "mashups" of different singers who had sung the same song. Their separate covers were mixed together as if they were collaborating. The prototype chorus featured all vocalists singing the entire song together, thus the name (see definition below). Later choruses were spliced to distribute singers into solos and smaller groups as well.

One of the definitions of "Chorus" listed by Dictionary.com:

1. a group of persons singing in unison.

In the youtaite community nowadays, a chorus is a general term for any collaboration involving at least three vocalists.