An outlier is a number in a set that is significantly different from the other numbers. It can be significantly more or less than most of the numbers in the set of data.
Sometimes outliers are as a result of error in measurement or recording. When outliers are as a result of error, they should be ignored.
Sometimes outliers are an important piece of information that shouldn't be ignored. For example, when one student does much better or much worse than the rest of the class on a test, the outlier should not be ignored.
Outliers may not always be obvious. If they aren't obvious, identifying them becomes a matter of choice.
Look at the following set of numbers:
78, 341, 48, 29, 56, 32, 79, 86, 73
In this set, the number 341 is significantly different than the rest, so it is an outlier.
Watch the following video on outliers.