T-Bill

Treasury bills (T-bills) are zero-coupon bonds that mature in one year or less. They are bought at a discount of the par value and, instead of paying a coupon interest, are eventually redeemed at that par value to create a positive yield to maturity. https://en.wikipedia.org/wiki/United_States_Treasury_security#Treasury_bill

United States Treasury securities are government debt instruments issued by the United States Department of the Treasury to finance government spending as an alternative to taxation. Treasury securities are often referred to simply as Treasurys.[1] Since 2012, U.S. government debt has been managed by the Bureau of the Fiscal Service, succeeding the Bureau of the Public Debt.

The risk-free interest rate is the rate of return of a hypothetical investment with scheduled payment(s) over a fixed period of time that is assumed to meet all payment obligations. Since the risk-free rate can be obtained with no risk, any other investment having some risk will have to have a higher rate of return in order to induce any investors to hold it. In practice, to infer the risk-free interest rate in a particular currency, market participants often choose the yield to maturity on a risk-free bond issued by a government of the same currency whose risks of default are so low as to be negligible. For example, the rate of return on T-bills is sometimes seen as the risk-free rate of return in US dollars. https://en.wikipedia.org/wiki/Risk-free_interest_rate