A major geological epoch of the earth's history - the second division of the Tertiary Period (Cenozoic Era) that began about 57.8 million years ago and ended about 36.6 million years ago. It follows the Paleocene Epoch and precedes the Oligocene Epoch. The Eocene is often divided into Early (57.8 to 52 million years ago), Middle (52 to 43.6 mya), and Late (43.6 to 36.6 mya) epochs. The name Eocene is derived from the Greek eos ("dawn") and refers to the dawn of recent life; during the Eocene all the major divisions or orders of modern mammals appeared.