In theory, the risk-free rate is the minimum return an investor should expect for any investment, as any amount of risk would not be tolerated unless the expected rate of return was greater than the risk-free rate.
In practice, however, the risk-free rate does not technically exist; even the safest investments carry a very small amount of risk. Thus, investors commonly use the interest rate on a three-month U.S. Treasury bill as a proxy for the risk-free rate because short-term government-issued securities have virtually zero risk of default.
Investment dictionary. Academic. 2012.