The Collaborative International Dictionary of English v.0.48:

Eocene \E"o*cene\, a. [Gr. 'hw`s daybreak, dawn + ? new,
   recent.] (Geol.)
   Pertaining to the first in time of the three subdivisions
   into which the Tertiary formation is divided by geologists,
   and alluding to the approximation in its life to that of the
   present era; as, Eocene deposits. -- n. The Eocene formation.
   --Lyell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Eocene
    n 1: from 58 million to 40 million years ago; presence of modern
         mammals [syn: Eocene, Eocene epoch]