This is Wikipedia's definition:
Central: Dufferin, Haliburton, Haliburton, Hastings, Kawartha Lakes, Muskoka, Northumberland, Parry Sound, Peterborough, Prince Edward, Simcoe
Eastern: Frontenac, Lanark, Leeds and Grenville, Lennox and Addington, Ottawa, Prescott and Russell, Renfrew, Stormont, Dundas and Glengarry
Golden Horseshoe: Durham, Halton, Hamilton, Niagara, Peel, Toronto, York
Northern: Algoma, Cochrane, Greater Sudbury, Kenora, Manitoulin, Nipissing, Rainy River, Thunder Bay, Timiskaming
Southwestern: Brantford/Brant, Bruce, Chatham-Kent, Elgin, Essex, Grey, Haldimand, Huron, Lambton, Middlesex, Norfolk, Oxford, Perth, Waterloo, Wellington
Of these, Hastings and Prince Edward strike me as definitely being Eastern Ontario.
There's also the postal code definition, with Eastern Ontario ("K") starting at Cobourg (Port Hope was part of Durham County and Cobourg in Northumberland when postal codes were established) and including Peterborough and Haliburton, and setting Muskoka and Parry Sound in Northern Ontario ("P"). Central Ontario is a large swath running east of Georgian Bay ("L", though "M" in Toronto) and from Georgian Bay west is southwestern Ontario ("N").
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