A data value which is unusual with respect to the group of data in which it is found. It may be a single isolated value far away from all the others, or a value which does not follow the general pattern of the rest. Most classical statistical techniques tend to be quite sensitive to outliers, so that it is important to be on the alert for them. Graphical techniques, particularly residual plots, are very helpful in detecting the presence of outliers.
Some of the newer Exploratory Data Analysis techniques and nonparametric procedures are much less sensitive to outliers (such procedures are said to be robust).