A fatal disease that causes the death of a character, whether major or minor. The character(s) that has died of Plot Device has/have bitten the dust in order to somehow advance the plot (ie: the plot would not move forward without their death)
To determine if the true cause of a character's death was indeed Plot Device, one needs to look at the events following the death and ask if the plot would have moved forward if the character hadn't died. If it would not have, the character has died of Plot Device.
(Plot Device was named so by the fact that anything that moves the plot forward is a plot device.)
Lily and James Potter (Harry Potter), L (Death Note), Shinobu Sensui (YuYu Hakusho), Press Tilton (Pendragon), Brom (Eragon), Mr. and Mrs. Wayne (Batman), and many more have died from Plot Device.
Unfortunately, Plot Device seems to be the true killer of many of the fan-favorites.