Definition and background:
A tempo marking indicating a quick duple meter with the half note rather than the quarter note getting the beat (2/2 rather than 4/4). Both the name and the sign are a vestige of mensural notation and of the proportions ( Tempus imperfectum diminutum). See also cut time. In cut-time; two beats per measure or the equivalent thereof. A duple tempo in which the half note gets the beat.
For more information on the music term "Alla breve" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary