Definition and background:
A choral setting of an English religious text similar to a motet, usually used in church with or without organ accompaniment.An anthem is a short vocal composition. in the church of England the word indicates such a composition often using a non-liturgical text (i.e. not part of the official service). A full anthem is for full choir, without soloists, while a verse anthem makes contrasting use of solo singers. both these forms flourished in the church of England from the late 16th century.
For more information on the music term "Anthem" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary