What is the definition of "ice age"?

An ice age is a long period of time in which the average temperature on Earth is greatly reduced. The reduction in the Earth's temperature results in an expansion of the continental ice sheets, polar ice sheets and mountain glaciers.

During an ice age, individual cold temperature changes in climate are referred to as glacial periods, while warming periods are known as interglacials. During interglacials the Earth's climate returns to normal, and the average temperature rises. The ice caps, ice sheets and glaciers begin to melt during the interglacial periods, but they do not melt completely. In Earth's history, there have been at least four known major ice ages.