A plot hole is an element within a story which does not make logical sense, or which illustrates something is missing in terms of character action. This can take a number of different forms and be as simple as characters knowing information that they should not, or not understanding something that is common knowledge. A more elaborate plot hole can be an event within a story that makes no sense, such as the sudden appearance of an item in a scene that was not previously introduced. The severity of this type of moment can be fairly minor, though in some instances it can serve to unravel the logic of an entire narrative.
There are a number of different ways in which a plot hole can develop in a story. These moments are often the result of a writer wanting to have a scene function a certain way, without establishing a logical reason behind it. If a writer thinks of a moment between two characters that is especially dramatic or appealing, then he or she might make it part of a story. This type of decision can create a plot hole, however, since the setup necessary for the scene to really make sense may not have been added.
One of the most common types of plot hole is an instance in which a character has knowledge that he or she should not reasonably have. Often, this is seen in movies or television programs when a new character arrives with answers to all of the questions already established in the story. This device allows a storyteller to provide answers without characters finding it through more elaborate means, but often ignores how the knowledgeable new addition found this information in the first place.
Similarly, a plot hole can develop when a character seems to lack common knowledge. If a reasonable audience member can determine something about the events of a story to come, then characters should be able to do so as well. Character ignorance can create dramatic irony within a story, but it can be detrimental to realism if taken too far.
The actual events within a story can often create a plot hole, especially if problems are resolved through convenience rather than a logical progression of events. Common examples of this include minor details like characters finding car keys in the visor or glove compartment of a vehicle. In the real world, most people do not keep a spare set of keys in such a convenient location, for fear of theft. When a plot hole like this takes place in a story, it may be so off-putting that it destroys realism for the audience.