Pronunciation: (ē'u-sēn"), [key] Geol.
noting or pertaining to an epoch of the Tertiary Period, occurring from 55 to 40 million years ago and characterized by the advent of the modern mammalian orders. See table under geologic time.
the Eocene Epoch or Series.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.