Acid

• (chemistry) A substance that produces H⁺(aq) ions in aqueous solution. Strong acids ionize completely or almost completely in dilute aqueous solution. Weak acids ionize only slightly.
• (biology) A substance that dissociates, releasing hydrogen (H⁺) ions but not hydroxyl (OH^-) ions; whose aqueous solution has a pH less than 7. Source: Curtis, Helena. 1968. Biology. New York, NY. Worth Publishers
• (geology)

1. A solution of pH less than 7.0 at 25 degrees C.
2. A substance containing hydrogen that may be replaced by metals with the formation of salts. CTD
   Source: Dictionary of Mining, Mineral, and Related Terms