The Efficient Frontier is a concept from Modern Portfolio Theory (MPT), introduced by Harry Markowitz in 1952 (Nobel Prize, 1990). It represents all possible portfolio combinations of assets that maximize expected return for a given level of risk — or equivalently, minimize risk for a given expected return.
Portfolios that lie on the frontier are called 'efficient' — you cannot improve them by simply swapping assets without either taking on more risk or accepting lower returns. Portfolios below the frontier are 'inefficient' because a better risk-return tradeoff exists.
The key insight is that combining assets with low or negative correlation reduces the overall portfolio risk below the weighted average of individual asset risks. This is the mathematical proof that diversification works.
On StressTest.pro, the Efficient Frontier is computed via Mean Variance Optimization using historical covariance matrices. The optimizer searches thousands of portfolio weight combinations to trace the frontier curve and identify the tangency portfolio (the highest Sharpe Ratio point on the curve).