What is an outlier?

The definition depends on the statistics used.

If you've calculated the standard deviation, any value that is over twice the standard deviation of the set of points is classified as an outlier.

If you've calculated the interquartile , any value that is 1.5(IQR) below the lower quartile or 1.5(IQR) above the upper quartile is classified as an outlier.

You can also test using many more complex methods, but those are the two most basic