The Collaborative International Dictionary of English v.0.48:

Antepenult \An`te*pe"nult\, Antepenultima \An`te*pe*nult"i*ma\,
   n. [L. antepaenultima (sc. syllaba) antepenultimate; ante
   before + paenultimus the last but one; paene almost + ultimus
   last.] (Pros.)
   The last syllable of a word except two, as -syl- in
   monosyllable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antepenultima
    n 1: the 3rd syllable of a word counting back from the end [syn:
         antepenult, antepenultima, antepenultimate]