ice age

noun2

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.

   • The earth is currently in the Late Cenozoic Ice Age, an ice age that began 33.9 million years ago with the Antarctic Ice Cap.
2. •
   (informal) Any of several glacial periods (within a scientific ice age) marked by episodes of extensive glaciation alternating with episodes of relative warmth (interglacial).

   • The cause of ice ages, Köppen decided, is to be found in cool summers, not brutal winters. — 2003