We say that the median is a resistant measure, whereas the mean is not a resistant measure. What's a resistant measure?
A resistant measure is one that is not influenced by outliers.
For example if we have an ordered list of numbers:
1, 3, 4, 5, 6, 8, 50
The mean is: 11
The median is 5
The mean in this case is larger than most of the numbers on the list because it is influenced so strongly by 50, in this case a strong outlier. The median would remain 5 even if the last number in the ordered list was much larger, as it simply provides the middle number in an ordered list of number.