Plotholes (also rendered plot hole) are those annoying gaps in a story where things happen without a logical reason. When a Plothole involves something essential to a story's outcome, it can hurt the believability severely for those who are bothered by it. They can be created either through bad characterization, inconsistent plotting, or the existence of something contradicted in canon.
Note that a Plothole is inherently a contradiction. A Plot element that is merely left unexplained is not a Plothole unless its occurrence is impossible according to the setting's rules.