The "efficient frontier" is a concept in modern portfolio theory that represents a set of optimal investment portfolios. These portfolios offer the highest expected return for a given level of risk or the lowest risk for a given level of expected return. Investors use the efficient frontier to make informed decisions about asset allocation. The efficient frontier is typically illustrated as a curve on a graph, where the x-axis represents risk (volatility) and the y-axis represents expected return. Portfolios that lie on this curve are considered efficient, while those below it are suboptimal. This concept is closely associated with the work of Harry Markowitz, who introduced it in the 1950s.