Source : Webster's Revised Unabridged Dictionary (1913)

Antacid \Ant*ac"id\, n. [Pref. anti- + acid.] (Med.)
   A remedy for acidity of the stomach, as an alkali or
   absorbent. -- a. Counteractive of acidity.

Source : WordNet®

antacid
     adj : acting to neutralize acid (especially in the stomach)
     n : an agent that counteracts or neutralizes acidity (especially
         in the stomach) [syn: {gastric antacid}, {alkalizer}, {alkaliser},
          {antiacid}]