The American Meteorological Society Glossary of Meteorology contains an entry for ice ages that reads, "A major interval of geologic time during which extensive ice sheets (continental glaciers) formed over many parts of the world.
The best known ice ages are 1) the Huronian in Canada, occurring very early in the Proterozoic era (2700–1800 million years ago); 2) the pre-Cambrian and early Cambrian, which occurred in the early Paleozoic era (about 540 million years ago) and left traces widely scattered over the world; and 3) the Permo-Carboniferous, occurring during the late Paleozoic era (from 290 million years ago), which was extensively developed on Gondwana, a large continent comprising what is now India, South America, Australia, Antarctica, Africa, and portions of Asia and North America. The term ice age is also applied to advances and retreats of glaciers during the Quaternary era"
This last bit, the Quaternary era ice ages, is the most recent period of major glaciations, and is most often discussed in analogy with anthropogenic climate change. The period from about 2 million years ago until 20,000 years ago, quasi-periodic ice ages dominate the climatic record. These glacial times are recorded in geologic records, such as stalactites and deep-sea sediment cores. To decipher those records, isotopic analysis is commonly employed. Most of the Quaternary ice ages are spaced by about 41,000 years, in conjunction with changes in Earth's obliquity. In the past 800,000 years, the ice ages were about 100,000 years long, punctuated by warm interglacial periods similar to the current Holocene.