Bedrooms are defined in a number of ways depending on the purpose of the regulation.
For the purposes of the Ontario Building Code (now available on line), there is the above-noted requirement for a window.
For the purposes of this conversation the (other) germane definition is found in the local development charges by-law, which tie the development charges rate to the number of bedrooms for apartment/condo (dwelling) units (more bedrooms = more people = greater need for services). The City of Toronto Development Charges By-law 275-2009 defines a bedroom to be "A room used or designed or intended for use as sleeping quarters but does not include a living room, dining room, kitchen or an area to be used as a den, study or other similar area". Municipalities use the definition as a means of capturing higher development charges on condominium and rental apartment developments, in part by trying to prevent developers, particularly in places like Waterloo where student housing is a big issue, from labelling rooms as anything other than a bedroom to avoid these higher charges.
The (former) City of Toronto Zoning By-law 438-86 on the other hand takes a narrower view of what a bedroom is, defining it as a "habitable room larger than 7 square metres, but does not include a living room, dining room or kitchen". The zoning regulations have more of a social than a revenue-generating purpose.
In terms of the question posed by nhincompoop above, there has to be a window for the room to be called a bedroom (Building Code), but it may be that calling the room a "den" is a clever means of avoiding a higher development charge rate. This begs the question of course on whether one can use a den as a sleeping quarters...
Blaney McMurtry LLP