Definition and background:
A performance style in which an ensemble is divided into two or more groups, performing alternately as separate groups and in unison. (1) an antiphonal chant origianlly used a musically interesting section sung by the choir (the antiphon) to frame a musically boring section (usually the recitational psalm tone). The antiphonal chants of the mass are the introit, offertory and communion. (2) an antiphonal chant was performed in alternation, with one half of the choir answering the other half. Psalms were often performed antiphonally, breaking either at the half- verse or at the line ending.
For more information on the music term "Antiphonal" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary