Any of a large class of sour-tasting substances whose aqueous solutions are capable of turning blue litmus indicators red, of reacting with and dissolving certain metals to form salts, and of reacting with bases or alkalis to form salts.

• 酸任何一类酸性的物质，它的水溶液可以使蓝色石蕊试纸变红，与金属反应或溶解金属时可形成盐，和碱反应亦可形成盐