Definition and background:
A hymn or prayer to the Virgin Mary, the words of which are taken from the Gospel of St. Luke. It was adapted for use in the Roman Catholic rite in the 16th century. Numerous polyphonic settings by Renaissance composers.
For more information on the music term "Ave maria" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary