Pleis·to·cene (pl??st?-s?n?)adj. Of, relating to, or being the epoch of geologic time from about 2.6 million to 12,000 years ago, the older of the two epochs of the Quaternary Period. It is characterized by the alternate appearance and recession of northern glaciation, the appearance and worldwide spread of hominins, and the extinction of numerous land mammals, such as the mammoths, mastodons, and saber-toothed tigers. See Table at geologic time.n. The Pleistocene Epoch.[Greek pleistos, most; see pel?- in Indo-European roots + -cene.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.Pleistocene (?pla?st??si?n) adj (Geological Science) of, denoting, or formed in the first epoch of the Quaternary period, which lasted for about 1 600 000 years. It was characterized by extensive glaciations of the N hemisphere and the evolutionary development of mann (Geological Science) the Pleistocene the Pleistocene epoch or rock series[C19: from Greek pleistos most + kainos recent]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Pleis?to?cene (?pla? st??sin) adj. 1. of or pertaining to the geologic epoch forming the earlier half of the Quaternary Period, beginning about two million years ago and ending ten thousand years ago, the time of the last Ice Age and the advent of modern humans. n. 2. the Pleistocene Epoch or Series. [1830?40;