"Acid rock" is loosely defined. Rock journalist Nik Cohn called it a "fairly meaningless phrase that got applied to any group, no matter what its style". It was originally used to describe the background music for acid trips in underground parties in the 1960s (e.g. the Merry Pranksters' "Acid Tests") and as a catchall term for the more eclectic Haight-Ashbury bands in San Francisco. The Grateful Dead's Jerry Garcia believed that acid rock is music you listen to while under the influence of acid, further stating that there is no real "psychedelic rock" and that it is Indian classical music and some Tibetan music "designed to expand consciousness".
Psychedelia was sometimes referred to as "acid rock". The latter label was applied to a pounding, hard rock
variant that evolved out of the mid-1960s garage-punk
movement. ... When rock began turning back to softer, roots-oriented sounds in late 1968, acid-rock bands mutated into heavy metal
—Frank Hoffman, Encyclopedia of Recorded Sound (2004)
The term is regularly deployed interchangeably with "psychedelic rock". According to Per Elias Drabløs, "acid rock is generally considered a subgenre of psychedelic rock", while Steve and Alan Freeman state the two are synonymous, and that "what is usually referred to as acid rock is generally the more extreme end of that genre". This would mean psychedelic rock that is heavier, louder, or harder.
As a hard rock variant of psychedelia, acid rock evolved from the 1960s garage punk movement, with many of its bands eventually transforming into heavy metal acts.[nb 1] Percussionist John Beck defines "acid rock" as synonymous with hard rock and heavy metal.[nb 2] The term eventually encompassed heavy, blues-based hard rock bands. Musicologist Steve Waksman wrote that "the distinction between acid rock, hard rock, and heavy metal can at some point never be more than tenuous".