Plot Contrivance
An occurrence in a story which is highly unlikely according to the pre-established story.
Alternativelyā€¦
Highly artificial plot progression or character interaction for the purpose of progressing the plot.
Elaboration
Basically, this is related to plot holes in that the plot hole can be considered a sub-variation of a plot contrivance. Iā€™m actually not sure about the definitions Iā€™ve given above, so I may change them in the future.
Plot contrivances occur when a payoff lacks adequate setup. Typically, theyā€™ll also appear or feel artificial, though thatā€™s not technically required for the definition.
What separates a contrivance from a hole is that the latter is impossible within a story, while the former is still somewhat possible, just incredibly unlikely.