interglacial adj.

word-forming element used freely in English, "between, among, during," from Latin inter (prep., adv.) "among, between, betwixt, in the midst of" (also used extensively as a prefix), from PIE *enter "between, among" (source also of Sanskrit antar, Old Persian antar "among, between," Greek entera (plural) "intestines," Old Irish eter, Old Welsh ithr "among, between," Gothic undar, Old English under "under"), a comparative of root *en "in."

A living prefix in English from 15c. and used with Germanic as well as Latinate words. Spelled entre- in French; most words borrowed into English in that form were re-spelled 16c. to conform with Latin except entertain, enterprise. In Latin, spelling shifted to intel- before -l-, hence intelligence, etc.

1650s, "cold, icy," from French glacial or directly from Latin glacialis "icy, frozen, full of ice," from glacies "ice," probably from a suffixed form of PIE root *gel- "cold; to freeze" (source also of Latin gelu "frost"). Geological sense "pertaining to glaciers" apparently was coined in 1846 by British naturalist Edward Forbes (1815-1854). Hence figurative sense "at an extremely slow rate," as of the advance of glaciers. Related: Glacially.