A glacial boundary is a line on a map representing the farthest advance of a glacier

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

 that has retreated. It generally refers to the extent of continental, rather than alpine, glaciers.

In the northern hemisphere, glaciers advanced from the north during the Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 epoch. The glacial boundary thus represents the farthest southern extent of the glaciers at that time.

In North America, the boundary from the most recent ice sheet passes through the states of New York, New Jersey, Pennsylvania, Ohio, Kentucky, Indiana, Illinois, Missouri, Kansas, Nebraska, South Dakota, Montana, and Washington. It also crosses the U.S.-Canadian border in several locations.

See Timeline of glaciation

Timeline of glaciation

There have been five known ice ages in the Earth's history, with the Earth experiencing the Quaternary Ice Age during the present time. Within ice ages, there exist periods of more severe glacial conditions and more temperate referred to as glacial periods and interglacial periods, respectively...

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See the U.S. National Atlas for a map of the boundary in North America.