The Greater Toronto Area, commonly referred to as the GTA, includes the city of Toronto and the regional municipalities of Durham, Halton, Peel, and York. In total, the region contains 25 urban, suburban, and rural municipalities. The Greater Toronto Area begins in Burlington in Halton Region to the west, and extends along Lake Ontario past downtown Toronto eastward to Clarington in Durham Region.

According to the 2021 census, the Census Metropolitan Area (CMA) of Toronto has a total population of 6.202 million residents, making it the nation's largest, and the 7th-largest in North America. However, the Greater Toronto Area, which is an economic area defined by the Government of Ontario,^{[citation needed]} includes communities that are not included in the CMA, as defined by Statistics Canada. Extrapolating the data for all 25 communities in the Greater Toronto Area from the 2021 Census, the total population for the economic region included 6,711,985 people.^{[citation needed]}

The Greater Toronto Area is a part of several larger areas in Southern Ontario. The area is also combined with the city of Hamilton to form a conurbation known as the Greater Toronto and Hamilton Area (GTHA). The GTHA combined with Niagara Region form the core of the Golden Horseshoe.

The term "Greater Toronto" was first used in writing as early as the 1900s although at the time, the term referred only to the old city of Toronto and to its immediate townships and villages, which became Metropolitan Toronto in 1954 and became the current city of Toronto in 1998. The use of the term involving the four surrounding regional municipalities came into formal use in the mid-1980s, when it was used in a widely discussed report on municipal governance restructuring in the region and was later made official as a provincial planning area. However, it did not come into everyday usage until the mid-to-late 1990s.

In 2006, the term began to be supplanted in the field of spatial planning as provincial policy increasingly began to refer to either the "Greater Toronto and Hamilton Area" (GTHA) or the still-broader "Greater Golden Horseshoe". The latter includes the Greater Toronto Area's satellite municipalities, such as Peterborough, Barrie, Guelph, Kitchener, Waterloo, Cambridge and Niagara Region. The GTA continues, however, to be in official use elsewhere in the Government of Ontario, such as the Ministry of Finance.

Some municipalities considered part of the GTA are not within the Toronto Census Metropolitan Area (CMA), which is smaller than the land area and population of the GTA planning area. For example, Oshawa is the centre of its own CMA, yet deemed part of the Greater Toronto Area, while other municipalities, such as New Tecumseth in southern Simcoe County and Mono Township in Dufferin County are included in the Toronto CMA but not in the GTA. These different border configurations result in the GTA's population being higher than the Toronto CMA by nearly one-half million people, often leading to confusion amongst people when trying to sort out Toronto's urban population.

Other nearby urban areas, such as Hamilton, Barrie, St. Catharines-Niagara or Kitchener-Waterloo, are not part of the GTA or the Toronto CMA, but form their own CMAs near the GTA. Ultimately, all the aforementioned places are part of the Greater Golden Horseshoe metropolitan region, an urban agglomeration, which is the fifth most populous in North America. It is part of the Great Lakes megalopolis, containing an estimated 59.1 million people in 2011.

The term "Greater Toronto and Hamilton Area" (GTHA) refers to the GTA, and the city of Hamilton, located along the western border of the Greater Toronto Area. The term has been adopted by several organizations, including Metrolinx and the Ministry of Energy) because of the growing commuter population in the combined region. The GTHA and the Regional Municipality of Niagara form the inner ring of the larger Greater Golden Horseshoe, an urban agglomeration and secondary region of Ontario.