Summary
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 busting the Moon
Post Crisis Superman defeating Dominus, a multiversal cosmic being
Hulk destroying a pocket universe with a thunderclap
Batman hurting The Spectre with a kick
Captain America hurting Onslaught
Thor containing a blast powerful enough to destroy 1/5th of the universe
Iceman defeating Oblivion