ice age

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A cold period marked by extensive glaciation.
• noun The most recent glacial period, which occurred during the Pleistocene Epoch.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of several cold periods in the history of the earth marked by episodes of extensive glaciation alternating with episodes of relative warmth.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any period of time during which glaciers covered a large part of the earth's surface