Definition and background:
the brass section of the orchestra includes metal instruments where the sound is produced by forcing air through a cup-shaped or conical mouthpiece. the brass section usually consists of trumpets, trombones and tuba and French horns
An alloy of copper and zinc, usually with two to one proportion and with yellow or golden coloration. Because of the addition of zinc and sometimes small amounts of other metals, brass is stronger and harder than copper, but it is also malleable. Inscribed brass plates with descriptive information have been used traditionally to label formal or academic religious, historical and portrait paintings. Source: Ralph Mayer, "A Dictionary of Art Terms and Techniques"
For more information on the music term "Brass" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary