ice age

Short definition from Wiktionary

Noun

1. 1
   A period of long-term reduction in the temperature of Earth's surface and atmosphere, resulting in the presence of major polar ice sheets that reach the ocean and calve icebergs.
2. 2
   Any of several glacial periods (within a scientific ice age) marked by episodes of extensive glaciation alternating with episodes of relative warmth (interglacial).
3. 3
   Alternative letter-case form of ice age.