n: A friend in which you are allowed sexual activity but no true relationship is invovled. Not a girlfriend or boyfriend.
The benefit is purely sexual. Not to be tied with feelings.

a friend with whom you may occassionally "hook up" or make out with, etc etc, and may do so without worries about commitment.

A concept exactly described and labeled "erotic friendship" by Milan Kundera in his classic novel "The Unbearable Lightness of Being", which impressed the literate world back in 1984. That is basically what that book is about: "friend with benefits" vs. "traditional". - And who wins? Well, let's say: both fail, but "traditional" does it with more grace.

a friend that you are not supposed to be commited to or attached to, that you fuck, or do other sexual activities with.

a person who one fucks on the side when not involved in a serious relationship. NOT to be confused with a boyfriend or girfriend, to whom (gasp!) commitment would be involved.

Friend with benefits is a Magic friend. The ideal relationship. The miss of compromise makes both be excellent friends and lovers.

a guy who is a girls friend. they aren't dating but they can have sex. the only time you can have sex with two people without considering it cheating.