standard equilibrium constant, K °, K

Synonym: thermodynamic equilibrium constant

Quantity defined by K ° = e − Δ r G ° / R T where Δ r G ° is the standard reaction Gibbs energy, R the gas constant and T the thermodynamic temperature. Some chemists prefer the name thermodynamic equilibrium constant and the symbol K.
Source:
Green Book, 2nd ed., p. 50
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 548