Abnormal return or AR measures the difference between the actual return a stock earns over a certain period and the return normally one expects to earn. A positive abnormal return means a stock performed better than the market, while a negative one indicates that the stock underperformed the market.It is the difference between the actual return of a security and the expected return.Abnormal returns are triggered by "events." Events like dividend announcements, bonus issues, rights issues, mergers, company's earnings announcements, etc. contribute to abnormal return.