It is rare to come across one, but recently we were all proven that it exists. Canonically hot is when a character is officially hot, as told by the creator or in other official sources that have not been corrected statements later on. When this is confirmed, it is likely true in real life as well.
The particular reason that this definition was created was because Nintendo of America president Reggie Fils-Aimé said himself, that Breath of the Wild's main character Link was hot.
"Link is canonically hot."
"When I make a book, at least one character will be canonically hot."