A data value that is unusually small or unusually large when
compared to the overall pattern of the values in the data set

An outlier.

Step-by-step explanation:

An outlier is a data value that is unusually small or unusually large when compared to the overall pattern of the values in the data set.

Simply stated, outliers lies outside most of the other values in a data set, thus, making them different from the other values.

An outlier may be as a result of experimental error or variations in the measurement.

For example, in the data set;

26, 25, 24, 2, 31, 90, 34, 33, 30, and 29. The outliers are 2 and 90.