An ice age is a period of long-term downturn in the temperature of Earth's climate, resulting in an increase of the continental ice sheets, polar ice sheets and mountain glaciers. Glaciologically, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres; by this description we are still in an ice age. More colloquially, when speaking of the most recent few million years, ice age is used to refer to colder periods with extensive ice sheets over the North American and European continents: in this sense, the last ice age ended about 10,000 years ago. This article will use the term ice age in the former, glaciological, sense; and use the term 'glacial periods' for colder periods the duration of ice ages and 'interglacial' for the warmer periods.
During the last few million years, there have been many glacial periods, occurring firstly at 40,000-year frequency but more recently at 100,000-year frequencies.