Of or belonging to the geologic time, rock series, or sedimentary deposits of the last epoch of the Tertiary Period, characterized by the appearance of distinctly modern animals. From thirteen million to two million years ago; there was a growth of mountains; a cooling of the climate; and more and larger mammals.

During this time the global climate became cooler and the number and expanse of grassslands and savannas increased greatly. This change in vegetation was accompanied by an increase in long-legged grazers. In the later part of the epoch, many of the species living in polar regions became extinct.

Coined from Greek pleion, and kainos or cene, "new".