Rf
From ACT Wiki
'Rf' is an abbreviation for the theoretically risk free rate of investment returns which can be earned on hypothetical investments, considered to be risk-free for modelling purposes.
For example, in the Capital asset pricing model.
Interest rate benchmarks
The term 'risk-free rates' is also used in the context of interest rate benchmarks.
This type of 'risk-free rate' is more often abbreviated 'RFR'.