Mi·o·cene (m???-s?n?)adj. Of, relating to, or being the epoch of geologic time from about 23 to 5.3 million years ago, the fourth epoch of the Tertiary Period. It is characterized by the spread of grasses and grazing mammals. See Table at geologic time.n. The Miocene Epoch.[Greek mei?n, less; see mei- 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.Miocene (?ma???si?n) adj (Geological Science) of, denoting, or formed in the fourth epoch of the Tertiary period, between the Oligocene and Pliocene epochs, which lasted for 19 million yearsn (Geological Science) the Miocene this epoch or rock series[C19: from Greek mei?n less + -cene]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Mi?o?cene (?ma? ??sin) Geol. adj. 1. noting or pertaining to an epoch of the Tertiary Period, occurring from 25 million to 10 million years ago, when grazing mammals became widespread. n. 2. the Miocene Epoch or Series. [1832; mio- (