Works of church music written to be sung as a single vocal line in free rhythm and a restricted scale, in a style developed for the medieval Latin liturgy. It is monodic and does not need instrumental accompaniment although this may be provided. Gregorian chant is named after Saint Gregory the Great (ca. 540-604) who is said to have standardized it. Gregorian chant refers to a Roman form of early plain chant as distinguished from the Ambrosian, Galliean, and Mozarabic chants, which were similar, but were gradually supplanted by it from the eighth to the eleventh century.

Source: Art & Architecture Thesaurus

 http://vocab.getty.edu/page/aat/300265207