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, Jefferson Airplane, New Riders of the Purple Sage 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.
Historically and more accurately, the term refers to the relationship between music and LSD, commonly called "acid". "Acid rock" can connote music recorded under the influence of LSD, or intended to be experienced in conjunction with LSD. These associations can be a matter of the musician's intention, the listener's intention, or even just a general perception on the part of the person using the term. Often the lyrics deal with drug-induced psychological themes as well as references to mind-altering techniques.
Characterized by an off-beat style, vivid imagery, and sometimes strange sound/musical effects (e.g. backward recorded music), acid rock often settles into a hypnotic groove wherein a listener (and possibly even the band) can "get lost" within a song.