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.)