A media tie-in is a book or story or comic book that's based on a property from another media. A Star Trek novel is a media tie-in. So's a Spider-Man novel or a CSI comic book or a Warcraft manga. *grin*
Tie-ins are often looked down on by the literary intelligentsia as lesser literature -- yet a Pulitzer goes to a book that's really no different from a Conan novel.
Hey, I don't know how many of them won major prizes before, but both tie-ins and fanfic have been respectable literary forms for ages, going back at least as far as Virgil's Aeneid which is, like, totally an Iliad fanfic and a blatant Marty Stu to boot.
Word, Scuba Girl. A few years ago I took a class on Arthurian literature, and some of it was flat out self-insertion fanfic. For example, we had the tale of Sir Whosit who was the only one who could drink from a magical horn of wine where even King Arthur failed, only to learn at the end of the story that it was written by Sir Whosit.
It occurred to me at that point that Literature is probably just that which survives, and "good" or "bad" becomes largely irrelevant in the end.