Progressive bluegrass

Progressive bluegrass is a type of bluegrass music which became popular in the 1960s and 1970s. The four important elements (not always all present) of progressive bluegrass are instrumentation (usually including electric instruments, drums, piano, and more), songs imported or styles imitated from other musical genres (like jazz, rock and others), non-traditional chord progressions, and lengthy "jam band"-style improvisation.
Bands such as Nickel Creek produce music in this style.