- (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