(Noun) This is a metaphorical
hell for men. It is typically initiated by a
guy asking out a girl and her saying "Oh, I only see you as a friend.".
One common example of this is girls
will reject a nice
guy, and date douche-bags, all while complaining to the nice guy and saying things like "Why can't all guys be like you?".
Again, this is only a stereotype.
It is possible to escape the friendzone, but it is not easy. It is a long and difficult road.