When I think of a riddle, I think of the questions between Gollum and Bilbo in The Hobbit, or the classic riddle of The Sphynx. With that in mind, I might define a riddle as "A description or set of clues that indicate a single noun." This meta question has a slightly different definition, maybe one that subsumes my own. Unfortunately, there's not an official description for that tag like there are for other.
It seems like most of the questions with the riddle tag are actually brainteasers and not riddles, by either definition. It's one of the more popular tags; I suspect people use it because they don't want to bother choosing a more accurate one. In any case, this meta question seems to think we should not allow riddle challenge questions.
So the tag is unclear, probably gets misused (but not for lack of a better tag), and is generally unwelcome, should we get rid of it? If not, what should we use it for? I suppose it's valuable if it's paired with a puzzle-creation-like tag, but I'm not sure how valuable that is in the face of the problems with it. If we are keeping it, should we be editing or closing questions that don't have another tag?