(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) any of a range of compounds, being hydrates of silica; they form silicates having complex polymeric structures

(chemistry) Any of various complex phenols extracted from the bark or galls of trees; used in tanning.

(biochemistry) The aldaric acid, 2,3 dihydroxy-succinic acid, that occurs naturally in many plants, in wine and in tamarind; it is used as the salts cream of tartar and Rochelle salt.

(chemistry) a weak acid, either of two hydrated forms of titanium dioxide, H₂TiO₃ or H₄TiO₄; used as a mordant

(chemistry) A poisonous unstable liquid, HSCN; its salts and esters, the thiocyanates, have a variety of uses.

(organic chemistry) Any of the three isomers ortho-, meta- and para- methylbenzoic acid, derived from toluene.

(inorganic chemistry) One of several systematic acid names for water, H₂O.

(inorganic chemistry) The weak acid, H₂SO₃, present in a solution of sulfur dioxide in water; it is a reducing agent and mild bleach.