My definition: bad, tacky, or dated movies. Hammy acting, bad scripts, what have you. Even YOU know it's a bad movie, but you love it anyway. OR, a movie that you think admitting to liking would ruin your reputation (like a chick flick for guys, etc). Basic rule: if you'd be embarrassed to admit you love the movie, it's a guilty pleasure.
My list off the top of my head:
Road House
Freddy Got Fingered (horrible movie, but it makes me laugh out loud every time)
Orgazmo
Titanic (I really should hate this movie with a passion, but try as I might to hate it, I'm a sucker)
The Village
The Quick and the Dead
Baseketball
Legends of the Fall (I almost listed this as one of my favorite movies, then was embarrassed to say it, so I guess it fits)
Freeway (only because I worry that digging this movie reveals that I'm twisted)
Coyote Ugly