Definition and background:
The family of wind instruments which are made of a brass or silver tube (either cylindrical bore or conical bore) that flares into a bell at the end, have cupped mouthpieces similar to that of a trumpet, and usually have valves or slides. The members of this family include the trumpet, horn, trombone, tuba and their families.
For more information on the music term "Brass instruments" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary