The difference between the return on a stock (or entire portfolio) and the performance of an index, such as the S&P; 500. The abnormal return is equal to the market return - the normal return. For example, a stock that provided a return of 10% over the same period of time in which an index provided a 6% return would have an abnormal return of 10% - 6% = 4%. If the abnormal return is negative then it has underperformed the index.