part song

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An unaccompanied secular, usually homophonic choral composition, especially of the 19th century.
2. n. An unaccompanied secular polyphonic song predating the madrigal.

Century Dictionary and Cyclopedia

1. n. In music, a vocal composition for two or more independent voices or parts; loosely, a glee or madrigal, and sometimes a round or catch. Part-songs are usually meant to be sung without accompaniment.

Wiktionary

1. n. music A form of unaccompanied, often homophonic, vocal composition usually for a few voices or a small choir

GNU Webster's 1913

1. n. a song in two or more (commonly four) distinct vocal parts.
2. n. A song with two or more voice parts.

WordNet 3.0

1. n. a song with two or more voice parts