in reference to the second epoch of the Tertiary Period, 1831, from eo- "earliest" + Latinized form of Greek kainos "new" (see -cene). Coined in English (along with Miocene and Pliocene) by the Rev. William Whewell, English polymath, and meant as "the dawn of the recent." As a noun from 1851.
It has occurred to me that [kainos] is a better word than [neos], and I propose for your terms, 1 acene, 2 eocene, 3 miocene, 4 pliocene. ... For eocene you might say spaniocene, but I like your eo better. Is not this shortest and best? [Whewell, letter to Lyell, Jan. 31, 1831]