I've heard it been referred to as postoperative delirium. My SO has spent a good amount of time in hospitals, but coming out surgery this last time she started to hallucinate. At first we all had a good time as she described these fantastical visions to her nurses and I as if she were watching something on TV. Later things started to get quite terrifying. She was convinced that the nurses were conspiring to kill her. Not a fun experience, but I've heard from many others that it's a fairly common one.