A slasher, imo, is a film in which a normally masked and somewhat human (not a werewolf or anything) killer murders several characters throughout the film one by one. A murder is normally seen at the very start or just after the credits, to get the audience on its side as it were. Lots of sex and nudity are normally present too. And, more often that not, there is always a 'final girl' - the last character left to defeat the killer, is almost always female.
That's what I would say anywho.
