The Collaborative International Dictionary of English v.0.48:

antacid \ant*ac"id\ ([a^]nt*[a^]s"[i^]d), antiacid
\an*ti*ac"id\([a^]n*t[-e]*[a^]s"[i^]d), n. [Pref. anti- + acid.]
   (Med.)
   A remedy for neutralizing acidity, especially of the stomach,
   as an alkali or absorbent. -- a. Counteractive of acidity.
   [1913 Webster + AS]

WordNet (r) 3.0 (2006):

antiacid
    n 1: an agent that counteracts or neutralizes acidity
         (especially in the stomach) [syn: antacid, gastric
         antacid, alkalizer, alkaliser, antiacid]