Bluegrass

noun 1. any of several grasses (genus Poa) of which some have bluish-green culms; especially Kentucky bluegrass 2. [from the Blue Grass Boys, performing group, from Bluegrass state, nickname of Kentucky] country music played on unamplified stringed instruments (as banjo, fiddle, guitar, and mandolin) and characterized by free improvisation and close usually high-pitched harmony

Merriam-Webster's Collegiate Dictionary