Michael Molinari @ Gamasutra: "It really shouldn't be a matter of opinion whether a game is a shmup or not. It either falls into the defined characteristics, or it doesn't. However, just like the many anomalies of the English language, so too do the rules of defining shmups (or any video game genre for that matter) delve into the realm of uncertainty.
First off, a "shoot 'em up" is more of what you do in the game, not what type of game it is. The same goes for "first-person shooter," which is simply a point of view and what you do.
Calling Resident Evil a "survival horror game" is halfway there, though the survival part is just a tacked-on noun (and honestly, there are hundreds of other games that require you to survive, but don't mention it in the genre). I'm not even a fan of calling games "role-playing games," as I naturally enjoy playing the role of whatever character I'm playing, even Dr. Mario."