From the point of view of mathematical statistics, risk-free assets refer to assets with zero variance or standard deviation of investment income. Of course, the covariance and correlation coefficient between the return on risk-free assets and the return on risk assets are also zero.
Theoretically, only fully indexed bonds issued by the central government with a maturity matching that of investors can be regarded as risk-free assets. In the real economy, there are very few securities in circulation that fully meet the above conditions. Therefore, in investment practice, risk-free assets are generally regarded as money market tools, such as treasury bill rate.