Chemistry. a compound usually having a sour taste and capable of neutralizing alkalis and reddening blue litmus paper, containing hydrogen that can be replaced by a metal or an electropositive group to form a salt, or containing an atom that can accept a pair of electrons from a base. Acids are proton donors that yield hydronium ions in water solution, or electron-pair acceptors that combine with electron-pair donors or bases.
a substance with a sour taste.
something, as a remark or piece of writing, that is sharp, sour, or ill-natured:
His criticism was pure acid.
Slang. (def 2).

belonging or pertaining to acids or the anhydrides of acids.
having only a part of the hydrogen of an acid replaced by a metal or its equivalent:
an acid phosphate.
having a pH value of less than 7.
Compare (def 4).

sharp or biting to the taste; tasting like vinegar; sour:
acid fruits.
sharp, biting, or ill-natured in mood, manner, etc.:
an acid remark; an acid wit.
Geology. containing much silica.
Metallurgy. noting, pertaining to, or made by a process in which the lining of the furnace, or the slag that is present, functions as an acid in high-temperature reactions in taking electrons from oxide ions: usually a siliceous material, as sand or ganister.
Compare (def 3).
put on the acid, Australian Slang. to importune someone, as for money, sexual favors, or confidential information.
Contemporary Examples

But unlike many other detractors of the dismal errand into Iraq, Danner leavens his acid judgments with firsthand reporting.
America’s Dark Side Chris Lehmann October 24, 2009

There was no acid in the bucket, just water mixed with some cleansers, which gave the sensation of burning.
Acid Attacks on Women Spread Terror in Iran IranWire October 17, 2014

Just this morning, McFadden reminded us, an acid attack was reported in Brooklyn, New York, mere miles from the Hudson Theatre.
Stealing Beauty: The Devastating Impact of Acid Violence Jesse Ellison March 10, 2011

Your acid experiences also seem to dovetail with expanding your musical consciousness.
Herbie Hancock Holds Forth David Yaffe November 7, 2014

Furthermore, there is not just a “pool” of acid in your stomach.
Quora Q: Does Drinking Water During Meals Help or Hinder the Digestive System? Quora Contributor January 29, 2014

Historical Examples

In the same instant, there came the sharp and acid twinge of planoform.
The Game of Rat and Dragon Cordwainer Smith

Of acid it would take 60 times the weight of the gas, or nearly 76 tons.
Flying Machines W.J. Jackman and Thos. H. Russell

Yellowish white flesh, crisp and beautiful flavor, from a mingling of the acid and saccharine.
Soil Culture J. H. Walden

That look was like the bite of acid that reveals the structure of crystal in metals.
The Martian Cabal Roman Frederick Starzl

Pass the acid liquor through the same small filter but collect the liquor apart.
A Textbook of Assaying: For the Use of Those Connected with Mines. Cornelius Beringer and John Jacob Beringer

any substance that dissociates in water to yield a sour corrosive solution containing hydrogen ions, having a pH of less than 7, and turning litmus red See also Lewis acid
a sour-tasting substance
a slang name for LSD

of, derived from, or containing acid: an acid radical
being or having the properties of an acid: sodium bicarbonate is an acid salt

sharp or sour in taste
cutting, sharp, or hurtful in speech, manner, etc; vitriolic; caustic
(of rain, snow, etc) containing pollutant acids in solution
(of igneous rocks) having a silica content of more than 60% of the total and containing at least one tenth quartz
(metallurgy) of or made by a process in which the furnace or converter is lined with an acid material: acid steel

1620s, “of the taste of vinegar,” from French acide (16c.) or directly from Latin acidus “sour, sharp,” adjective of state from acere “to be sour,” from PIE root *ak- “sharp, pointed” (see acrid). Figurative use from 1775; applied to intense colors from 1916. Acid test is American English, 1892, from the frontier days, when gold was distinguished from similar metals by application of nitric acid. Acid rain is first recorded 1859 in reference to England.

1690s, from acid (adj.). Slang meaning “LSD-25” first recorded 1966 (see LSD).

When I was on acid I would see things that looked like beams of light, and I would hear things that sounded an awful lot like car horns. [Mitch Hedberg, 1968-2005, U.S. stand-up comic]

Acid rock (type played by or listen to by people using LSD) is also from 1966; acid house dance music style is 1988, probably from acid in the hallucinogenic sense + house “dance club DJ music style.”

acid ac·id (ās’ĭd)

Any of a large class of sour-tasting substances whose aqueous solutions are capable of turning blue litmus indicators red, of reacting with and dissolving certain metals to form salts, and of reacting with bases or alkalis to form salts.

A substance that ionizes in solution to give the positive ion of the solvent.

A substance capable of yielding hydrogen ions.

A proton donor.

An electron acceptor.

A molecule or ion that can combine with another by forming a covalent bond with two electrons of the other.

A substance having a sour taste.

See LSD.


Of or relating to an acid.

Having a high concentration of acid.

Having a sour taste.

Any of a class of compounds that form hydrogen ions when dissolved in water, and whose aqueous solutions react with bases and certain metals to form salts. Acids turn blue litmus paper red and have a pH of less than 7. Their aqueous solutions have a sour taste. Compare base.

acidic adjective

A sour-tasting material (usually in a solution) that dissolves metals and other materials. Technically, a material that produces positive ions in solution. An acid is the opposite of a base and has a pH of 0 to 7. A given amount of an acid added to the same amount of a base neutralizes the base, producing water and a salt. Common vinegar, for example, is a weak solution of acetic acid.

Note: Figuratively, acid applies to anything sour or biting; for example, an “acid wit” is sharp and unpleasant.


: an acid party


The hallucinogen LSD, which is chemically an acid; a (Narcotics)

Related Terms

battery acid
A mnemonic for the properties a transaction should have to satisfy the Object Management Group Transaction Service specifications. A transaction should be Atomic, its result should be Consistent, Isolated (independent of other transactions) and Durable (its effect should be permanent).
The Transaction Service specifications which part of the Object Services, an adjunct to the CORBA specifications.
aircraft identification


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