A person (usually a woman) that performs sexual favours for money. Not entirely dissimilar to a girlfriend, wheras prostitutes requires cash, a girlfriend requires being taken out for meals, bought cars and shoes ergo a prostitute is a girlfriend that requires less effort. Also are less likely to say "oh so your friends are more important than me?" everytime you wanna go for a fuckin' drink with your mates.
Bill: "Hey Ben, is this your prostitute?"
Ben: "No Bill, this is my girlfriend"
Bill: "OK OK, Wanna come out with the guys for a couple of drinks?
Ben: "Nah, my girlfriend wont let me"
Bill: "*tut*, buy that prostitute some shoes"