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 this framework 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 was popularized by economist Harry Markowitz in the 1950s.