Of substance B at temperature T, ยต B o ( T ), is the value of the chemical potential under specified standard conditions (in the standard state).
Source:
Green Book, 2nd ed., p. 53 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
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 (https://doi.org/10.1351/pac199466030533)