Group of assets dominated by at least one other portfolio under the mean variance rule. For example, if A has both lower return and higher volatility than B, we say A is dominated by B.
A portfolio that provides too low a return for the risk. That is, an inefficient portfolio has taken on so much risk that the return is not worth the effort. In Markowitz portfolio theory, an inefficient portfolio is graphically represented as any portfolio that does not follow the efficient frontier, or the set of portfolios that provide the highest return at each different level of risk.