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Wiktionary: chorus line
Noun
chorus line (plural chorus lines)
1. A line of performers in a revue or other show, dancing in unison.

(Oxford English Dictionary)
chorus line n. Theatre (originally U.S.) a group of (esp. female) dancers who perform synchronized routines, typically in a row, sometimes also singing; also in extended use.
1898 Evening Herald (Syracuse, N.Y.) 19 Mar. 9/1 That same pretty girl..stepped out of the chorus line one night to have a try at one of the principal parts.
1972 Times 9 Feb. 14/5 The chorus line did a French-style can can, flipping their skirts to expose their thighs and panties.

Wikipedia: A Chorus Line
A Chorus Line is a 1975 musical with music by Marvin Hamlisch, lyrics by Edward Kleban, and a book by James Kirkwood Jr. and Nicholas Dante.

Set on the bare stage of a Broadway theater, the musical is centered on seventeen Broadway dancers auditioning for spots on a chorus line. A Chorus Line provides a glimpse into the personalities of the performers and the choreographer, as they describe the events that have shaped their lives and their decisions to become dancers.