ice age

jääkausi scientific: A period of long-term reduction in the temperature of Earth's surface and atmosphere, resulting in the presence of polar ice sheets and alpine glaciers.
colloquial: Any of several glacial periods (within a scientific ice age) marked by episodes of extensive glaciation alternating with episodes of relative warmth (interglacial).
2003, (w), A Short History of Nearly Everything, BCA 2003, p. 377:

The cause of ice ages, Köppen decided, is to be found in cool summers, not brutal winters.