In statistics an outlier is a distribution point (for example, a number or a score) that is much further away from any other distribution points. Outliers can skew measurements so that the results are not representative of the actual numbers. An outlier can occur through natural variability or because of a problem with the test.
An example would be a very hard test given to a small class of 5 students. The test scores were 50,50,50,50, and 100. The average test score would be 60 although almost everyone scored less than that. The score of 100 is an outlier. Either the student is very smart and did well on the test (natural variability) or they cheated (a problem with the test).