Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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