Definition and background:
A form of country & western (C&W) music thought to have originated before WW II but actually developed during the mid 1940s. Fans, DJs, and record companies began using the term " Bluegrass " to describe a sound associated with the music of Bill Monroe and his Bluegrass Boys. Monroe is ofted referred to as "The Father of Bluegrass." The term refers to Kentucky (the Bluegrass state), Bill Monroe's home state. Typically performed by a " string " band consisting of violin, mandolin, guitar, 5-string banjo, and string bass.
For more information on the music term "Bluegrass" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary