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. LSD ( 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 alkaline ( 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 basic ( def 3 ).
put on the acid, Australian Slang. to importune someone, as for money, sexual favors, or confidential information.

1620–30; < Latin acidus sour, akin to ācer sharp, acētum vinegar, acescent, acicula

acidly, adverb
acidness, noun
nonacid, noun, adjective
preacid, adjective
preacidness, noun
semiacid, adjective

acerbic, acid, acrid.

7. acerbic, stinging, vitriolic, tart. Acid, astringent are terms used figuratively of wit or humor. Acid suggests a sharp, biting, or ill-natured quality: an acid joke about an opponent. Astringent connotes severity but usually also a bracing quality, as of something applied with curative intent: astringent criticism. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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World English Dictionary
acid (ˈæsɪd)
1.  See also Lewis acid 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
2.  a sour-tasting substance
3.  a slang name for LSD
4.  chem
 a.  of, derived from, or containing acid: an acid radical
 b.  being or having the properties of an acid: sodium bicarbonate is an acid salt
5.  sharp or sour in taste
6.  cutting, sharp, or hurtful in speech, manner, etc; vitriolic; caustic
7.  (of rain, snow, etc) containing pollutant acids in solution
8.  (of igneous rocks) having a silica content of more than 60% of the total and containing at least one tenth quartz
9.  metallurgy of or made by a process in which the furnace or converter is lined with an acid material: acid steel
[C17: (first used by Francis Bacon): from French acide or Latin acidus, from acēre to be sour or sharp]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1620s, from Fr. acide, from L. acidus "sour," adj. of state from acere "to be sour," from PIE base *ak- "sharp, pointed" (see acrid). Applied to intense colors from 1916. Slang meaning "LSD-25" first recorded 1966 (see LSD); 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 test is Amer.Eng., 1892, from the frontier days, when gold was distinguished from similar metals by application of nitric acid. Acid rain is first recorded 1859 in ref. to England. Adj. acidic is attested from 1877, originally in geology.
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]
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

acid ac·id (ās'ĭd)

  1. 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.

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

  3. A substance capable of yielding hydrogen ions.

  4. A proton donor.

  5. An electron acceptor.

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

  7. A substance having a sour taste.

  8. See LSD.

  1. Of or relating to an acid.

  2. Having a high concentration of acid.

  3. Having a sour taste.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
acid   (ās'ĭd)  Pronunciation Key 
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
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

acid definition

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.
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
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Abbreviations & Acronyms
aircraft identification
The American Heritage® Abbreviations Dictionary, Third Edition
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