A term used to describe the returns generated by a given security or portfolio over a period of time that is different from the expected rate of return. The expected rate of return is the estimated return based on an asset pricing model, using a long run historical average or multiple valuation.
Taobiz explains Abnormal Return
An abnormal return can be either a good or bad thing, as it is merely a summary of how the actual returns differ from the predicted return. For example, earning 30% in a mutual fund that is expected to average 10% per year would create a positive abnormal return of 20%. If, on the other hand, the actual return was 5%, this would generate a negative abnormal return of 5%.