The efficient frontier is a concept in modern portfolio theory introduced by Harry Markowitz and others. A combination of assets, i.e. a portfolio, is referred to as "efficient" if it has the best possible expected level of return for its level of risk (usually proxied by the standard deviation of the portfolio's return).[1] Here, every possible combination of risky assets, without including any holdings of the risk-free asset, can be plotted in risk-expected return space, and the collection of all such possible portfolios defines a region in this space. The upward-sloped part of the left boundary of this region, a hyperbola, is then called the "efficient frontier". For more information see modern portfolio theory.
The efficient frontier is the positively sloped portion of the opportunity set that offers the highest expected return for a given level of risk. The efficient frontier lies at the top of the opportunity set or the feasible set.