An Acid

1. A sour substance.
2. (chemistry) Any of several classes of compound having the following properties:
   1. Any of a class of water-soluble compounds, having sour taste, that turn blue litmus red, and react with some metals to liberate hydrogen, and with bases to form salts.
   2. Any compound that easily donates protons; a Brønsted acid
   3. Any compound that can accept a pair of electrons to form a covalent bond; a Lewis acid
3. (uncountable, slang) LSD, lysergic acid diethylamide.

Antonyms

• alkali
• base