A woman, usually very hot (but not always), who will essentially trade sex for gifts, some form of help/assistance or in order to social climb. These are the women who expect to be (well) taken care of by wealthy men (because they think they are worth it) - women who hang around VIP sections at clubs, etc. They aren't on the clock like a hooker and they aren't directly paid like a hooker, but they don't spread their legs or give you the time of day unless they are getting something or they think you will get something from you....thus making them halfway hookers.
That halfway hooker I met at in the VIP section of the club was expensive - I had to take her shopping, out to dinner a few times and away for a weekend just to get her into bed!
That girl is looking for sugardaddy - what a halfway hooker!