Definition and background:
A term used in American popular music to describe a continuous heavy accent on beats 2 and 4 in jazz and rock and roll music.
A consistent rhythm that stresses beats 2 and 4 in common time. In other "common" time signatures, the backbeat will land elsewhere. For example, the backbeat lands on 4 and 10 in 12/8 time.
For more information on the music term "Backbeat" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary