Efficient Frontier.
A definition, in plain English — with the books that teach it.
What it means
The efficient frontier is the set of portfolios that offer the highest expected return for each level of risk (standard deviation), or equivalently the lowest risk for each level of expected return. Developed by Harry Markowitz, it is the core output of Modern Portfolio Theory. Portfolios on the efficient frontier are said to be "mean-variance efficient"; any portfolio below or to the right of the frontier is suboptimal — it either carries too much risk for its return or leaves return on the table for the risk taken.
Example
An investor holds a 100% equity portfolio with 10% expected return and 18% standard deviation. By adding 20% bonds, the optimizer finds a portfolio on the efficient frontier with 9.2% expected return but only 14% standard deviation — nearly identical return with meaningfully less volatility.
