Its a girl that is virtually a girlfriend in every way shape and form, however the title girlfriend is still lacking.

Piece of ass that will not leave.

A girl that a guy isn't supposed to have more than one of, but does anyway.

a girl who takes all ur money all ur effort allur mind and in the end leaves you with nothing but the cothes on ur bak

A female with whom you exclusively have sexual relations. Unless it's an open relationship, in which case, see fuck buddy, cock holster, or Janelle.

a complete moneysucking bitch who takes ownership upon your balls.