acid

American  

[as-id] / ˈæs ɪd /

noun

1. 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.

2. a substance with a sour taste.

3. something, as a remark or piece of writing, that is sharp, sour, or ill-natured.

   His criticism was pure acid.

4. Slang. LSD.

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adjective

1. Chemistry.

   1. belonging or pertaining to acids or the anhydrides of acids.

   2. having only a part of the hydrogen of an acid replaced by a metal or its equivalent.

      an acid phosphate.

   3. having a pH value of less than 7.

2. sharp or biting to the taste; tasting like vinegar; sour.

   acid fruits.

3. sharp, biting, or ill-natured in mood, manner, etc..

   an acid remark; an acid wit.

   Synonyms:

   tart, vitriolic, biting, acerbic

4. Geology. containing much silica.

5. 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.

idioms

1. put on the acid, to importune someone, as for money, sexual favors, or confidential information.

acid British  

/ ˈæsɪd /

noun

1. 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

2. a sour-tasting substance

3. a slang name for LSD

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adjective

1. chem

   1. of, derived from, or containing acid

      an acid radical

   2. being or having the properties of an acid

      sodium bicarbonate is an acid salt

2. sharp or sour in taste

3. cutting, sharp, or hurtful in speech, manner, etc; vitriolic; caustic

4. (of rain, snow, etc) containing pollutant acids in solution

5. (of igneous rocks) having a silica content of more than 60% of the total and containing at least one tenth quartz

6. metallurgy of or made by a process in which the furnace or converter is lined with an acid material

   acid steel

acid Scientific  

/ ăs′ĭd /

1. 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.

2. Compare base

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acid Cultural  

1. 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.

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Usage

What does acid mean? In science, an acid is a sour-tasting substance that releases hydrogen ions when added to water. Acids will turn litmus to a red color and have a pH lower than 7. An acid is any substance that will release hydrogen ions when mixed with water. The amount of hydrogen ions that a substance releases is measured on the pH scale, which ranges from 0 to 14. The lower the pH number is, the more hydrogen ions that are being released. Water has a pH of 7, which is neutral on the pH scale. All acids have a pH lower than 7. A pH greater than 7 indicates an alkali, or base. Acids also have a sour taste, such as the acid found in lemons. A final quality that all acids share is that when they come into contact with litmus paper, they will turn the paper red. Acids are all around us and are used for a wide variety of purposes. Some common acids you can find in your house include lactic acid (in milk), ascorbic acid (in citrus fruits), and acetic acid (in vinegar).

Related Words

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.

Discover More

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

Other Word Forms

• acidly
  adverb
• acidness
  noun
• acidy
  adjective
• nonacid
  noun
• preacid
  adjective
• preacidness
  noun
• semiacid
  adjective

Etymology

Origin of acid

First recorded in 1620–30; from Latin acidus “sour,” akin to ācer “sharp,” acētum “vinegar”; cf. acescent, acicula

Explanation

In technical terms, acid is a substance that can react with a base to make a salt. In practical terms, acid is the stuff you want to keep off your hands, or any part of you, because it will eat away your skin. Not all acids will eat skin away. The citric acid in oranges and other citrus fruits gives them their tasty tang. There is a hallucinogenic street drug called acid that eats away at the user’s sense of reality. Words sometimes have the qualities of acid. In acidic speech, words feel corrosive to the listener’s sensibilities. This one’s perhaps the most frequent and most destructive of all.

Example Sentences

Plus, the trigger used to reverse the shape of the contorted molecule in order to release its energy was hydrochloric acid – a highly corrosive substance that must be neutralised after use.

From BBC • May 7, 2026

Trump's spokeswoman -- who is best known for her acid put-downs of journalists -- posted a picture of herself in a nursery, cradling her baby daughter to her chest.

From Barron's • May 7, 2026

Researchers from Kindai University and partner institutions found that arginine, a naturally occurring amino acid, can reduce the buildup of harmful Aβ proteins in animal models of Alzheimer's.

From Science Daily • May 4, 2026

It also has unusual habitats like ffridd, a Welsh term for a type of upland ecosystem common in Wales, that is made up of a combination of habitats including bog, acid grassland, and bracken.

From BBC • Apr. 27, 2026

In words belonging to chemistry derivatives in -ic denote the acid containing most oxygen, when more than one is formed: as nitric acid.

From New Word-Analysis by William Swinton

Related Words

• acerbic
• biting
• caustic
• mordant
• nasty
• sarcastic
• stinging
• trenchant
• vitriolic