Outlier Detection with Parametric and Non-Parametric methods

An Outlier is an observation or point that is distant from other observations/points. But, how would you quantify the distance of an observation from other observations to qualify it as an outlier. Outliers are also referred to as observations whose probability to occur is low. But, again, what constitutes low?? There are parametric methods and non-parametric methods that are employed to identify outliers.