What does acid mean?
An acid is a chemical substance defined by its ability to donate protons (hydrogen ions) in solution or, under broader definitions, to accept pairs of electrons. Acids characteristically taste sour, corrode certain metals, turn litmus paper red, and neutralize bases — properties familiar from everyday examples like vinegar, lemon juice, and stomach acid. The word also functions as an adjective describing anything sour, corrosive, or sharply cutting: acid rain damages forests, while an acid remark is one delivered with biting sarcasm. Chemists distinguish many specific acids, from sulfuric acid used in industry to the amino acids that build proteins in living cells. Derived forms such as 'acidic' and 'acidify' extend the word's reach into science and cooking alike. Both precise and vividly figurative, 'acid' bridges laboratory terminology and everyday language, making it one of the most versatile words in English.
A chemical substance that dissolves in water, has a pH below 7, donates protons or accepts electron pairs, and typically tastes sour and corrodes metals.
"Sulfuric acid is widely used in batteries and industrial manufacturing."
In chemistry, the exact definition depends on the theoretical framework: Arrhenius acids produce hydrogen ions in water, Brønsted–Lowry acids donate protons, and Lewis acids accept electron pairs.
Having the properties of an acid, or tasting sour; figuratively, sharp, biting, or sarcastic in tone.
"The soil was too acid for the hydrangeas to bloom blue."
The figurative sense ('acid wit') is more literary than colloquial.
Standard plural; commonly used when discussing classes of acids (e.g., amino acids, fatty acids).
"Citrus fruits contain several different organic acids."
The word 'acid' comes from the Latin acidus, meaning sour — the same root that gave us 'acetum,' vinegar, one of humanity's oldest acids.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of acid
The word 'acid' derives from the Latin adjective 'acidus,' meaning sour or tart, which itself traces back to the Proto-Indo-European root *ak-, signifying sharpness or pointedness. It entered English in the early 17th century, initially as an adjective describing sour tastes before becoming established as a noun for chemical substances. The same root gave rise to a family of related words including 'acrid,' 'acute,' and 'acumen,' all conveying sharpness in some form. Notably, Lavoisier's mistaken belief that oxygen was essential to all acids led Humphry Davy to name chlorine-derived compounds accordingly, embedding the root in chemical nomenclature.
Related word forms
How acid is actually used
As an adjective, 'acid' can describe both literal taste/chemistry ('acid soil') and figurative sharpness of manner ('an acid wit'), the latter being somewhat literary. In slang contexts referring to LSD, register is informal and often subcultural. In chemistry, 'acid' is a precise technical term whose definition varies by theory (Arrhenius, Brønsted–Lowry, Lewis).
Easily confused with acid
Acrid means sharply unpleasant in smell or taste (like smoke), while acid refers to a class of chemical substances or a sour taste.
In chemistry, a base is the opposite of an acid — it accepts protons rather than donating them and turns litmus blue.