Statistic measuring symmetry of a distribution.
Positive skewness indicates that extremely high values are relatively more common (right tail is fat), while negative skewness means that extremely low values are more common (left tail is fat).
Normal distribution has zero skewness (it is perfectly symmetric).
Most asset returns (including most stocks) show negative skewness: Market sometimes crashes down (less frequent but sometimes very extreme negative returns), but rarely crashes up (up moves tend to be more frequent but more orderly and moderate in size).