The Collaborative International Dictionary of English v.0.48:

Penult \Pe"nult\, n. [Abbreviated fr. penultima.] (Gram. &
   Pros.)
   The last syllable but one of a word; the syllable preceding
   the final one.
   [1913 Webster]

WordNet (r) 3.0 (2006):

penult
    n 1: the next to last syllable in a word [syn: penult,
         penultima, penultimate]