 InvestHub.com's Finance Dictionary and Glossary of Investment Terms efficient portfolio Definition 1.
A portfolio that provides the greatest expected return for a given level of risk, or equivalently, the lowest risk for a given expected return. also called optimal portfolio.  Definition 2.
A portfolio that provides the greatest expected return for a given level of risk (i.e., standard deviation), or, equivalently, the lowest risk for a given expected return. 

