Definition and background:
An instrument of copper or brass, similar to the cornet, but higher and more piercing in pitch. Formerly it was equipped with keys or valves, but now exists only in natural form and is used in military field music. A hunting horn.
A brasswind instrument with approximately conical bore profile, small enough to be a portable signalling instrument
For more information on the music term "Bugle" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary