equilibrium constant

Quantity characterizing the equilibrium of a chemical reaction and defined by an expression of the type Kx = ΠB.xB(νB), where νB is the stoichiometric number of a reactant (negative) or product (positive) for the reaction and x stands for a quantity which can be the equilibrium value either of pressure, fugacity, amount concentration, amount fraction, molality, relative activity or reciprocal absolute activity defining the pressure based, fugacity based, concentration based, amount fraction based, molality based, relative activity based or standard equilibrium constant (then denoted K°), respectively.
Green Book, 2nd ed., p. 50 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2187 (https://doi.org/10.1351/pac199062112167)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 972 (https://doi.org/10.1351/pac199668040957)