https://doi.org/10.1351/goldbook.S05908
Of substance B at temperature T, μBo(T), is the value of the chemical potential under specified standard conditions (in the standard state).
Source:
Green Book, 2nd ed., p. 53 [Terms] [Book]
See also:
PAC, 1994, 66, 533. (Standard quantities in chemical thermodynamics. Fugacities, activities and equilibrium constants for pure and mixed phases (IUPAC Recommendations 1994)) on page 536 [Terms] [Paper]