Definition of "Acid" :

noun: street name for lysergic acid diethylamide

noun: any of various water-soluble compounds having a sour taste and capable of turning litmus red and reacting with a base to form a salt

adjective: having the characteristics of an acid

"An acid reaction."

adjective: harsh or corrosive in tone

"A barrage of acid comments."

adjective: being sour to the taste