A man who is somewhere between a friend and a boyfriend. This can be a temporary state from which he will eventually become a boyfriend or a permanent state of more than friendship. Even if you have only hooked up with him once, if you have crossed that line, he's a man-friend.
That guy you keep trying to convince people is not your boyfriend: he's your man-friend.
A male significant other that possesses a higher level of maturity than the typical "boyfriend". It is usually a term used by women that have very high standards in a relationship and do not have the patience to tolerate immature bullshit.
"Is Julie still dating whats-his-face?"
"Nah dude, she upgraded to a Manfriend."
"Wow that is one classy fellow....definitely Manfriend material."