Ontario is a province located in the east-central part of the Canada, the largest by population and second largest (after Quebec) in total area. Ontario is bordered by the provinces of Manitoba to the west, Quebec to the east, and the United States states of Michigan, New York, Ohio, Pennsylvania and Minnesota.

Most of Ontario's borders with the United States are natural, starting at the Lake of the Woods and continuing through the four Great Lakes: Superior, Huron (which includes Georgian Bay), Erie, and Ontario (for which the province is named), then along the Saint Lawrence River near Cornwall. Ontario is also the only Canadian Province that borders the Great Lakes.

The capital of Ontario is Toronto, the largest city in Canada. Ottawa, the capital of Canada, is located in Ontario as well. The 2006 Census counted 12,160,282 residents in Ontario, which accounted for 38.5% of the national population.

The province takes its name from Lake Ontario, which is thought to be derived from ontarí:io, a Huron word meaning "great lake", or possibly skanadario which means "beautiful water" in Iroquoian.

Along with New Brunswick, Nova Scotia and Quebec, Ontario is one of the four original provinces of Canada when the nation was formed on July 1, 1867 by the British North America Act.[8] Ontario is Canada's leading manufacturing province accounting for 52 per cent of the total national manufacturing shipments.