Any substance that seems to defy normal laws of physics for the convenience of plot in literature or film. This can take any form necessary to advance or enhance a plot and oftentimes is central to the plot its self.
Kyrptonite in the Superman series. "Magic" Bullets in movies. Firearms that never seem to run out of ammunition could be said to be filled with Plotium-made bullets, as there is a Conservation of Matter defying amount of them per magazine.
Action Hero's are usually almost entirely made out of Plotium.
Usage: "The main character's shirt must be made of Plotium because it deflected bullets even though it's just a cotton tee shirt!"