n. A quality a man (or less commonly woman) is said to have when they are found attractive, but for no obvious or discernible reason. They may not be physically attractive, smart or possess any typical desirable qualities, but are nonetheless alluring. Possession of the Snape Factor is often subjective, and it is rare for two people to agree about it.
The phrase originates from the character Severus Snape, of the ubiquitous Harry Potter series, who is dark, snide and almost ghoulish in appearace, but is considered attractive by many nonetheless. He has the Snape Factor.
Girl A: I don't know why, but he's kind of cute, somehow...
Girl B: Are you crazy? He just chased a squirrel down the street with holy water!
Girl A: Yeah, but-- I don't know, he just has the Snape Factor.