The Collaborative International Dictionary of English v.0.48:

Pliocene \Pli"o*cene\, n. (Geol.)
   The Pliocene period or deposits.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pliocene \Pli"o*cene\, a. [Written also pleiocene.] [Gr. ? more
   + ? new, recent.] (Geol.)
   Of, pertaining to, or characterizing, the most recent
   division of the Tertiary age.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Pliocene
    n 1: from 13 million to 2 million years ago; growth of
         mountains; cooling of climate; more and larger mammals
         [syn: Pliocene, Pliocene epoch]