1. Noun, Person:
A famous character in the American Pie
series of movies. Also, each of the American Pie Presents movies (Straight to DVD spinoffs) feature some member of the Stifler family.
Band Camp featured his brother, Matt, who we see briefly in the first American Pie movie when Steve steals his computer, to watch Nadia and Jim.
Both the Naked Mile and Beta House featured Erik Stifler, Steve Stifler's cousin.
2. Adjective. Used to describe someone who is sexually active, rude, crass, but loveable.