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 of The Doors, The Jimi Hendrix Experience, Pink Floyd, Iron Butterfly, Big Brother & The Holding Company, Cream, The Velvet Underground, Jefferson Airplane, Ultimate Spinach, New Riders of the Purple Sage, Blue Cheer, Quicksilver Messenger Service, The Great Society, 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.

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 June 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".

An acid (from the Latin acidus/acēre meaning sour) is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger. Chemicals or substances having the property of an acid are said to be acidic.

Common examples of acids include acetic acid (in vinegar), sulfuric acid (used in car batteries), and tartaric acid (used in baking). As these three examples show, acids can be solutions, liquids, or solids. Gases such as hydrogen chloride can be acids as well. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid.

There are three common definitions for acids: the Arrhenius definition, the Brønsted-Lowry definition, and the Lewis definition. The Arrhenius definition states that acids are substances which increase the concentration of hydronium ions (H₃O⁺) in solution. The Brønsted-Lowry definition is an expansion: an acid is a substance which can act as a proton donor. Most acids encountered in everyday life are aqueous solutions, or can be dissolved in water, and these two definitions are most relevant. The reason why pHs of acids are less than 7 is that the concentration of hydronium ions is greater than 10⁻⁷moles per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acids thus have pHs of less than 7. By the Brønsted-Lowry definition, any compound which can easily be deprotonated can be considered an acid. Examples include alcohols and amines which contain O-H or N-H fragments.

Rock usually refers to:

It can also refer to: