[‚ē·kwə′lib·rē·əm ‚vā·pə·rə′zā·shən ‚rā·shō] (physical chemistry) In a liquid-vapor equilibrium mixture, the ratio of the mole fraction of a component in the vapor phase (y) to the mole fraction of the same component in the liquid phase (x), or y/x = K (the K factor). Also known as equilibrium ratio.