Abnormal return refers to the return of a security or portfolio of securities that deviates from the market average, such as an index or other benchmark. This can be either a negative deviation ("underperformance") or a positive one ("outperformance").

Abnormal returns are often associated with specific events or market developments, such as news of a merger, acquisition, or legal proceedings that directly affect a particular security.

Understanding is essential in financial analysis to understand the impact of such events on returns.