Outlier
An observation point that is distant from other observation, due perhaps to variability in the measurement, or perhaps an indication of experimental error.
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Origin
The word outlier combines the prefix "out-" with "lier" (one who lies), with the earliest documented use dating to 1606. Originally applied to geology and other fields, the term developed statistical meaning in the 1900s. By 1865, the term was established enough that geologists coined "inlier" as its antonym. While the word predates modern statistics by centuries, its adoption into statistical terminology occurred as the field formalized in the late 19th and early 20th centuries, reflecting the need to describe data points that deviate significantly from patterns.
Updated February 22, 2026