definition of antacid
antacid in Medicine. antacid antÂ·acÂ·id (Änt-Äs'Ä­d) or anÂ·tiÂ·acÂ·id (Än'tÄ-Äs'Ä­d, Än'tÄ«-) adj. Counteracting or neutralizing acidity, especially of the stomach. n. A substance, such as sodium bicarbonate, that neutralizes acid.