1. (n.)Ontario a province in S Canada, bordering on the Great Lakes. 11,407,700; 412,582 sq. mi. (1,068,585 sq. km). Cap.:Toronto. Abbr.: ON, Ont.
2. Ontario Lake, a lake between the northeastern U.S. and S Canada, between New York and Ontario: the smallest of the Great Lakes. 7540 sq. mi. (19,530 sq. km).
3. Ontario a city in SW California, E of Los Angeles. 144,854.
Definition of 'Ontario'
Princeton's WordNet
1. (noun)Lake Ontario, Ontario the smallest of the Great Lakes
1. Ontario third largest, most populous, richest, and most important province of Canada, lies N. of the great lakes between Quebec and Manitoba, and is thrice the size of Great Britain; the surface is mostly undulating; there are many small lakes, the chief rivers flow eastward to join the Ottawa; agriculture is the chief industry, enormous crops of wheat, maize, and other cereals are raised; stock-rearing and dairy-farming are important; the climate is subject to less extremes than that of Quebec, but the winter is still severe; there are richmineral deposits, especially of iron, copper, lead, and silver, petroleum and salt; manufactures of agricultural implements, hardware, textiles, and leather are carried on; Toronto (181) is the largest town, Ottawa (44) is the capital of the Dominion, Hamilton (49) an important railway centre; the prosperity of the province is largely promoted by the magnificent waterways, lakes, rivers, and canals with which it is furnished. Founded by loyalists from the United States after the Declaration of Independence, the province was constituted in 1791 as Upper Canada, united to Quebec or LowerCanada in 1840, it received its presentname on the federation of Canada in 1867; education in it is free and well conducted; there are many colleges and universities; municipal and provincial government is enlightened and well organised; the prevalent religious faith is Protestant.
Definition of 'Ontario'
U.S. National Library of Medicine
1. Ontario A province of Canada lying between the provinces of Manitoba and Quebec. Its capital is Toronto. It takes its name from LakeOntario which is said to represent the Iroquois oniatariio, beautiful lake. (From Webster's New Geographical Dictionary, 1988, p892 & Room, Brewer's Dictionary of Names, 1992, p391)
Definitions of Ontario 
