In a data set, any extreme value, either very large or very small compared to the rest of the data, is said to be a(n)

An outlier is an observation lying far from the bulk of the data, whether unusually large or small.

Exceedance names how often a value crosses a threshold, not the extreme value itself.

Extreme is a loose descriptor, not the technical statistical term sought here.

Outsider has no standing as a measurement concept.

Outliers can distort the mean and inflate the variance.

They are often detected using boxplots or the interquartile range rule.