What is an outlier?

An outlier is an observation that is numerically distant from the rest of the data.  When reviewing a box plot, an outlier is defined as a data point that is located outside the whiskers of the box plot.

For example, outside 1.5 times the interquartile range above the upper quartile and below the lower quartile (Q1 – 1.5 * IQR or Q3 + 1.5 * IQR). The next Figure show how outliers are represented in a box-plot