The Collaborative International Dictionary of English v.0.48:

Miocene \Mi"o*cene\ (m[imac]"[-o]*s[=e]n), prop. a. [Gr. mei`wn
   less + kaino`s new, fresh, recent.] (Geol.)
   Of or pertaining to the middle division of the Tertiary. --
   n. The Miocene period. See Chart of Geology.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Miocene
    n 1: from 25 million to 13 million years ago; appearance of
         grazing mammals [syn: Miocene, Miocene epoch]