Chosen value of pressure denoted by p ⦵ or p°. In 1982 IUPAC recommended the value 10E5 Pa, but prior to 1982 the value 101325 Pa (= 1 atm) was usually used.
Green Book, 2nd ed., p. 54 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
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)