1
: a sour substance
specifically
: any of various typically water-soluble and sour compounds that in solution are capable of reacting with a base (see base entry 1 sense 6a) to form a salt, redden litmus, and have a pH less than 7, that are hydrogen-containing molecules or ions able to give up a proton to a base, or that are substances able to accept an unshared pair of electrons from a base
2
: something incisive, biting, or sarcastic
a social satire dripping with acid
3
: lsd
acidy
adjective
ˈa-sə-dē
1
a
: sour, sharp, or biting to the taste
an acid flavor
b
: sharp, biting, or sour in manner, disposition, or nature
an acid individual
an acid personality
c
: sharply clear, discerning, or pointed
an acid wit
acid criticism
d
: piercingly intense and often jarring
acid yellow
2
a
: of, relating to, or being an acid
also
: having the reactions or characteristics of an acid
acid soil
an acid solution
b
of salts and esters
: derived by partial exchange of replaceable hydrogen
acid sodium carbonate NaHCO3
c
: containing or involving the use of an acid (as in manufacture)
an acid bath
d
: marked by or resulting from an abnormally high concentration of acid
acid indigestion
3
: relating to or made by a process in which the furnace is lined with acidic material and an acidic slag is used
acid steel
4
: rich in silica
acid rocks
acidly
adverb
acidness
noun
Examples of acid in a Sentence
Noun
the acids in your stomach
the kinds of acid found in your stomach
Adjective
He washes his windows with an acid solution of vinegar and water.
with her acid personality, my aunt could be relied upon to bring some much-needed balance to the holiday gush
Recent Examples on the Web
Pauleanna Reid, Forbes.com, 9 July 2025
The almost-too-salty fries and crisp coleslaw add to the lip-puckering barrage of sodium and acid, while the rémoulade soothes your palate.
—Bradley Hohulin, IndyStar, 2 July 2025
Samantha Conti, Footwear News, 11 June 2025
Fluoride also replaces minerals lost from teeth due to acid breakdown, according to the Centers for Disease Control and Prevention.
—Mary Kekatos, ABC News, 3 June 2025
See All Example Sentences for acid
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Noun
The deodorant is infused with probiotics and includes ingredients like niacinamide and mandelic acid to support the skin barrier, even tone, and help balance odor, without relying on aluminum.
—Pauleanna Reid, Forbes.com, 9 July 2025
The almost-too-salty fries and crisp coleslaw add to the lip-puckering barrage of sodium and acid, while the rémoulade soothes your palate.
—Bradley Hohulin, IndyStar, 2 July 2025
Adjective
The floors are done in chestnut-toned wood, with acid green carpets made from bamboo silk, while the VVIP suite has its own private sitting room.
—Samantha Conti, Footwear News, 11 June 2025
Fluoride also replaces minerals lost from teeth due to acid breakdown, according to the Centers for Disease Control and Prevention.
—Mary Kekatos, ABC News, 3 June 2025
See All Example Sentences for acid
Cite this Entry
“Acid.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/acid. Accessed 20 Jul. 2025.
Copy CitationKids Definition
acid
2 of 2 noun1
: a sour substance
2
: a compound that usually dissolves in water, has a sour taste, reacts with a base to form a salt, and turns litmus paper red
3
: lsd
Medical Definition
acid
1 of 2 noun1
: a sour substance
specifically
: any of various typically water-soluble and sour compounds that in solution are capable of reacting with a base (see base sense 3a) to form a salt, redden litmus, and have a pH less than 7, that are hydrogen-containing molecules or ions able to give up a proton to a base, or that are substances able to accept an unshared pair of electrons from a base
2
: lsd
acid
2 of 2 adjective
ac·id
ˈas-əd 
1
: sour, sharp, or biting to the taste
2
a
: of, relating to, or being an acid
also
: having the reactions or characteristics of an acid
an acid solution
b
of salts and esters
: derived by partial exchange of replaceable hydrogen
acid sodium carbonate NaHCO3
c
: marked by or resulting from an abnormally high concentration of acid
acid indigestion
see also acid reflux
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