Definition and background:
A Catholic liturgical chant with a prose text associated with psalmody sung by two choirs in alternation, often part of roman catholic and Greek orthodox church services. may be sung as responses between a single voice and a group of voices, or between two different groups of singers. the effect of this back-and-forth is described as antiphonal. It is usually a response or refrain to a psalm or canticle verses. The response made by one part of the choir to another, or one by the congregation to the priest in a Catholic service; also, alternate singing.A musically interesting section of chant which is sung by a choir; the text and music were intended to serve as a frame to a psalm verse (or a series of psalm verses), introducing and following it. The term is used sometimes to mean sacred song (e.g. Marian antiphons).
For more information on the music term "Antiphon" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary