An outlier in math is a value in a collection of data points that lies outside of the range of the other data points. The name is derived from the data point "lying outside" of the other given values.
An example of an outlier can be seen in a group of data points taken from some athletes' performances: if there was one athlete that performed much more poorly than the other athletes, his performance is the outlier of the group.
Outliers of a data set most seriously affect the mean. The mean is reached by adding the data points together, then dividing the sum by the number of data points.