Other forms: ice ages
A long era of reduced temperatures on the earth, low enough to form new glaciers and polar ice sheets, is an ice age. There have been at least five ice ages over our planet's history.
The earliest ice age scientists know about dates from more than two billion years ago. All known ice ages have lasted millions of years, with periods of warming occurring during each era, during which glaciers retreat. We are actually living in the Pleistocene Ice Age right now, though we're in the midst of an interglacial period of warmth that's been exacerbated by human activity.