What is Abnormal Return?

Legal Definition
In finance, an abnormal return is the difference between the actual return of a security and the expected return. Abnormal returns are sometimes triggered by "events." Events can include mergers, dividend announcements, company earning announcements, interest rate increases, lawsuits, etc. all of which can contribute to an abnormal return. Events in finance can typically be classified as information or occurrences that have not already been priced by the market.
-- Wikipedia
Legal Definition
A periodic return on stocks traded in an investment portfolio should there be more or less than average returned. Also called an excess return.