The gradual distortion or even disintegration of a world and its characters during its odyssey from original source material to movie to TV movie then to television series then to video game and finally to licensed derivative work. The dramatic equivalent of photocopying a photocopy of a photocopy.
Every step away from the original property involves new input from multiple directions which dilutes and changes the flavor and behavior of the story. When handled well, Adaptation Decay can be minimized, and each generation of the process will remain reasonably faithful to the original. Handled poorly, and the TV series version of a favorite novel will look like a completely different product that just happens to have some of the same names.