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
Lana Bortolot, Forbes.com, 30 Apr. 2025
Heartburn Heartburn or acid reflux occurs when the valve connecting your esophagus and your stomach doesn’t keep your stomach acid where it’s supposed to be, according to the National Institute of Diabetes and Digestive and Kidney Diseases.
—Sarah Klein, Time, 29 Apr. 2025
John Mariani, Forbes.com, 18 Apr. 2025
The acid content of these foods irritates your child's skin, causing redness that can look like a rash.6
Viruses and Bacterial Infections
Certain viral or bacterial infections may cause multiple symptoms around the mouth, including a rash.
—Wendy Wisner, Health, 18 Apr. 2025
See All Example Sentences for acid
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Noun
The Schloss Johannisberg Kabinett deftly balances the sweetness of ripe fleshy fruit with acid and structure to deliver a finely finessed wine that, despite that refinement, shows energy and intensity.
—Lana Bortolot, Forbes.com, 30 Apr. 2025
Heartburn Heartburn or acid reflux occurs when the valve connecting your esophagus and your stomach doesn’t keep your stomach acid where it’s supposed to be, according to the National Institute of Diabetes and Digestive and Kidney Diseases.
—Sarah Klein, Time, 29 Apr. 2025
Adjective
The vintage was a cool one, allowing the grapes to mature slowly in autumn to balance sweet and acid components.
—John Mariani, Forbes.com, 18 Apr. 2025
The acid content of these foods irritates your child's skin, causing redness that can look like a rash.6
Viruses and Bacterial Infections
Certain viral or bacterial infections may cause multiple symptoms around the mouth, including a rash.
—Wendy Wisner, Health, 18 Apr. 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 10 May. 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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