Acid rock

Acid rock is a form of psychedelic rock, which is characterized with long instrumental solos, few (if any) lyrics and musical improvisation. Tom Wolfe describes the LSD-influenced music of The Jimi Hendrix Experience, The Doors, Cream, Jefferson Airplane, New Riders of the Purple Sage, Stone Garden and the Grateful Dead as "acid rock" in his book about Ken Kesey and the Acid Tests, The Electric Kool-Aid Acid Test.

"Acid rock" also refers to the subset of psychedelic rock bands that were part of, or were influenced by, the San Francisco Sound, and which played loud, "heavy" music featuring long improvised solos.

Source : http://en.wikipedia.org/wiki/Acid_rock