An Etymological Dictionary of Astronomy and Astrophysics

A substance that releases hydrogen ions to form a solution with a pH of less than 7, reacts with a base to form a salt, and turns blue litmus red.

See also: From Fr. acide, from L. acidus “sour,” adj. of state from acere “to be sour,” acer “sharp, pungent, bitter;” from PIE base *ak- “sharp, pointed.”