standard pressure

Chosen value of pressure denoted by $p ⦵$ or $p °$ . In 1982 IUPAC recommended the value $10 5 Pa$ , but prior to 1982 the value $101 325 Pa$ (= $1 atm$ ) was usually used.

Source:

Green Book, 2nd ed., p. 54

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