Risk free rate of return
From ACT Wiki
The theoretical rate of return which can be earned on hypothetical investments which are considered to be risk-free for modelling purposes.
Historically, the rates of return on certain types of domestic central government debt were considered to be a close enough proxy for such hypothetical risk-free investments.
In the modern era, domestic central government debt is no longer considered to be risk-free for this purpose, nor for a number of other purposes for which it was historically considered to be risk-free.