Grateful Dead album cover.
Acid rock is a genre of rock music, and a subgenre of psychedelic rock, which emerged in the 1960s and is associated with the use of psychoactive substances, notably LSD. It is characterized by a repetitive beat, heavy amplification, instrumental improvisation, new sound technologies, light shows, long jams, and lyrics suggestive of psychedelic experiences.
The experimentation in acid rock was further removed from the rock and Blues base of earlier psychedelic music, yet not as heavy as its close cousin heavy psych. Acid rock still features the extended instrumental jamming and improvisation of the psychedelic era, with more emphasis on 'wah' effects than the distorted 'fuzz' of hard psych, and a willingness to use more subdued textures. The term has largely fallen out of favor since the mid-1970s, as bands of the style either disbanded or adopted the traits of heavy psych.
Prominent acid rock bands were Jefferson Airplane, The Doors, The Grateful Dead, and The Jimi Hendrix Experience.