An Outlier is an event or incident that is considered to be completely and irreconcilably inconsistent with a character, entity, group, or series' normal displayed level of power. Outliers are generally regarded as unusable in some forums debates. However, all effort should be made to try to reconcile outliers with other canon information, and only the most extreme examples should be classed as completely unusable. Often there is disagreement on exactly what constitutes an outlier, and things that are considered an outlier by some might not be considered to be one by everyone. Careful judgement should be used in all cases.
Examples of outliers:
Master Roshi destroying the Moon.
Dormammu defeating Multi-Eternity.
Post Crisis Superman defeating Dominus, a multiversal cosmic being.
The Hulk being defeated by an ordinary python snake, or severely harmed by regular gorillas, and Thor being rendered unconscious by a shot from a handgun.