An ice age is a period of long-term reduction in the temperature of Earth's climate, resulting in an expansion 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 definition we are still in an ice age (because the Greenland and Antarctic ice sheets still exist).
More colloquially, when speaking of the last few million years, ice age is used to refer to colder periods with extensive ice sheets over the North American and Eurasian continents: in this sense, the most recent ice age ended about 10,000 years ago.
There are three main types of evidence for ice ages: geological, chemical, and paleontological.
Geological evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratics.
Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret.
It took some time for the current theory to be worked out.
There have been at least four major ice ages in the Earth's past.
Outside these periods, the Earth seems to have been ice-free even in high latitudes.