A person (usually independent and intelligent) who you leave to have a sexual codependent relationship with someone else.
The person you've known for years that goes on the back burner for your significant other.
A place holder until you find someone to sleep with.
The third wheel.
Girlfriend: Hey let's go grab a bite to eat.
Boyfriend: Alright sure, lets go.
Boyfriend: You mind if I invite John?
Girlfriend: Who's that?
Boyfriend: He's just a friend of mine.
Girlfriend: Well I only made reservations for two at "insert restaurant".
Ex two: Forever alone.