(chemistry) An antibacterial agent obtained from lichens of the genus Usnea.

(chemistry) the solid hydrated form of tungsten oxide, H₂WO₄; used in textiles, and in yellow glazes

(chemistry) A short chain aliphatic carboxylic acid, CH₃(CH₂)₃COOH, obtained from the roots of valerian; used in perfume, flavourings, pharmaceuticals

(biochemistry) a bicyclic heterocyclic phenolic compound, formed in the body by the metabolism of protein and excreted in the urine (or in faeces, as in birds and insects)

Rain which is unusually acidic (pH of less than the natural range of 5 to 6); caused mainly by atmospheric pollution with sulphur dioxide, carbon dioxide and nitrogen compounds.

(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.