The Collaborative International Dictionary of English v.0.48:

Oligocene \Ol"i*go*cene\, a. [Oligo- + Gr. ? new, recent.]
   (Geol.)
   Of, pertaining to, or designating, certain strata which
   occupy an intermediate position between the Eocene and
   Miocene periods. -- n. The Oligocene period. See the Chart of
   Geology.
   [1913 Webster] Oligochaeta

WordNet (r) 3.0 (2006):

Oligocene
    n 1: from 40 million to 25 million years ago; appearance of
         sabertoothed cats [syn: Oligocene, Oligocene epoch]