What are outliers?

A set of data may include scores that are extremely different from the rest of the group causing suspicion; scores that are remarkably different from the majority are called outliers.
For a set of numerical data, any value that is markedly smaller or larger than other values. Mathematically, outliers are considered any number that is more than 1.5 times the interquartile range away from the median. For example, in the data set {3, 5, 4, 4, 6, 2, 25, 5, 6, 2} the value of 25 is an outlier.

Basically the one that doesn't belong.

Basically, the outlier is the number that

stands out.So let's say we have these numbers: 2,3,1,17. 17 is our outlier. Why? Because, it's obvious.17 is separate away from the all the other numbers. An outlier is an element of a data set that distinctly stands out from the rest of the data.

Read more: http://wiki.answers.com/What_is_an_outlier_in_math#ixzz1FRclyotO