Definition of: acid

(as′id) adjective
1. Sharp and biting to the taste, as vinegar; sour.
2. Pertaining to, yielding, like, or reacting like, an acid.
3. Acidic.
4. Sharp-tempered; biting.
—noun
1. Any sour substance.
2a Any of a class of compounds that in aqueous solution turns blue litmus red and reacts with bases and with certain metals to form salts. b A compound that dissociates in water to yield hydrogen ions. c A compound that dissociates in a solvent to produce the positive ion of the solvent. d A molecule or ion that can attach itself to another molecule or ion by a covalent bond with an unshared pair of electrons.
3. Slang. LSD. See synonyms under BITTER. [<L acidus sharp, sour]
— ac′id·ly adverb
—ac′id·ness noun