Noun | 1. | factor of proportionality - the constant value of the ratio of two proportional quantities x and y; usually written y = kx, where k is the factor of proportionality constant - a number representing a quantity assumed to have a fixed value in a specified mathematical context; "the velocity of light is a constant" Planck's constant, h - the constant of proportionality relating the energy of a photon to its frequency; approximately 6.626 x 10^-34 joule-second factor - any of the numbers (or symbols) that form a product when multiplied together |