A plot hole is any noticeable failure in the internal logic of a narrative, based on the information the audience already possesses about the story universe within which that narrative is set.
If, for example, a story about time travel features a stipulation that the bootstrap paradox is not possible, and a character later creates a stable bootstrap paradox, with no explanation for why this was possible, then it is a plot hole.
An example of something that is not a plot hole is a MacGuffin: if the entire story is about characters who are fighting over a briefcase, it is not a plot hole that we never learn what is inside the briefcase. This does not disrupt the internal logic of the story; it is simply a gap in the information available to the audience.
Deus ex machina is also not a plot hole; if a previously unintroduced outside force has a significant impact on the course of the narrative, without contradicting the logic of the narrative, then no plot hole has occurred.
Retcon is likewise not a plot hole, provided the alteration to continuity is explained adequately using the internal logic of the story universe.
Numerous websites and organizations are dedicated to identifying plot holes and other errors in narratives, especially in film. Cinema Sins is one of these; Movie Mistakes is another.
Iron Noder 2015, 18/30