(histology, of bacteria) Retaining stain when treated with an acidic solution.

(figuratively) A rigorous test or appraisal of the quality or worth of something. / (historical) In the 1960s, a party based around the use and advocacy of the psychedelic drug LSD ("acid").

(chemistry) an unsaturated aliphatic carboxylic acid, CH₃.CH:CH.CH:CH.COOH, found in Rowan berries, and used as a preservative

(chemistry) a class of strong organic acids, only known as their salts in which the negative charge is stabilized by resonance.

(biochemistry) A naturally occurring organic compound, 3,4,5-Trihydroxy-1-cyclohexene-1-carboxylic Acid, extracted commercially from star anise; it is the base of very many natural products and is used in the production of the antiviral drug Tamiflu.

(chemistry) dihydroxycinnamic acid, obtained from certain umbelliferous plants

(chemistry) a complex mixture of organic acids produced by the decomposition of vegetable matter

(inorganic chemistry) The amphoteric aluminium hydroxide, Al(OH)₃; its salts are aluminates.

(chemistry) any of a class of organic sulphur-containing acids of general formula ROCS₂H

(chemistry, dated) heptanoic acid