As pointed out, the definition changes from place to place. It generally means a room that is not a normal part of the house (such as a porch, sunroom, etc....) that has sufficient aclilmiatization to allow it to be used year round. Whether that entails heat, cooling, insulation, or operable windows/screens depends on just where you are (in Michigan I suspect some heat source and operable windows is all that matters).
The real LEGAL question is why are you asking? If you're quibbling about something that is in a real estate listing, it's probably spurious. Listings are just advertisements, and errors there are not going to be actionable. You're expected to due more due diligence on things than relying on the puffery in the ad.