A line created from the risk-reward graph, comprised of optimal portfolios. 
Investopedia Says:
The optimal portfolios plotted along the curve have the highest expected return possible for the given amount of risk.
Related Links:
See why investors today still follow this set of principles to reduce risk and increase returns through diversification. Modern Portfolio Theory: An Overview
Diversification? Optimal portfolio theory? Read this tutorial and these and other financial concepts will be made clear. Financial Concepts
