Definition and background:
A graceful, stately court dance of the early Renaissance (c.1400-1600) without the rapid steps and leaps of other dances of the era. This dance was the precursor of the pavane, the music to it was generally improvised over a cantus firmus.
For more information on the music term "Basse dance" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary