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1.
A number of singers on the Japanese Nico Nico Douga site whose individual versions of the same song have been edited together in such a way that it appears that they are performing the song as one united chorus.

The song is usually (but not always) a vocaloid or anime song with a video backdrop, often with captions and/or icons identifying the individual singers.
Search for "Nico Nico Chorus" on Youtube for examples.
by greymousie January 17, 2011