Modern portfolio theory
Also: MPT
The framework for building portfolios by balancing risk and return through diversification.
Modern portfolio theory shows that an asset's contribution to a portfolio matters more than its return in isolation. Diversifying across imperfectly correlated assets improves the risk-adjusted result.
The enduring lesson is that an asset should be judged by what it does to the whole portfolio, not in isolation. Its assumptions are fragile, but the core insight, that diversification is close to a free lunch, remains one of the most useful in investing.