In statistics, an outlier is an observation point that is distant from other observations.
In statistics, an outlier is an observation point that is distant from other observations. An outlier may be due to variability in the measurement or it may indicate experimental error; the latter are sometimes excluded from the data set. Wikipedia
Outliers are not necessarily bad or wrong, nor do they need to be removed from data for further analysis. However, outliers (of which there can be more than one in any set of data) indicate that some data at least appear to differ from the bulk of the dataset, suggesting they should be individually examined and understood. Also, some statistical procedures are sensitive to outliers: this means that removal of one or more outliers could substantially change the conclusions of those procedures.