Acid rock is a form of psychedelic rock, which is characterized by long instrumental solos, few (if any) lyrics, and musical improvisation. Tom Wolfe describes the LSD-influenced music ofthe Jimi Hendrix Experience, Pink Floyd, the Doors, Iron Butterfly, Big Brother & the Holding Company, Cream, Jefferson Airplane, Ultimate Spinach, Blue Cheer, Quicksilver Messenger Service, the Great Society, Deep Purple 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.The Jimi Hendrix Experience
Acid rock got its name because it served as "background" music for acid trips in underground parties in the 1960s (e.g. the Merry Pranksters' "Acid Tests"). ("Acid" is a slang term for LSD.) In an interview with Rolling Stone, Jerry Garcia quoted Grateful Dead band member Phil Lesh stating, "acid rock is what you listen to when you are high on acid." Garcia further stated there is no real psychedelic rock and that it is Indian classical music and some Tibetan music that are examples of music "designed to expand consciousness." The term "acid rock" is generally equivalent to psychedelic rock. Rolling Stone magazine includes early Pink Floyd as "acid-rock". In July 1967 Time magazine wrote, "From jukeboxes and transistors across the nation pulses the turned-on sound of acid-rock groups: the Jefferson Airplane, the Doors, Moby Grape". In 1968 Life magazine referred to the Doors as the "Kings of Acid Rock". In 1969,Playboy Magazine referred to Led Zeppelin as "acid rock".
When hard rock and heavy metal became prominent in the early and mid-1970s, the phrase "acid rock" was sometimes mistakenly applied to these genres. Over time, these bands such as Alice Cooper fell in under the term "heavy metal" which replaced "acid rock" for these styles of music.