kinetic theory

:  either of two theories in physics based on the fact that the minute particles of a substance are in vigorous motion:a :  a theory that the temperature of a substance increases with an increase in either the average kinetic energy of the particles or the average potential energy of separation (as in fusion) of the particles or in both when heat is added —called also kinetic theory of heatb :  a theory that the particles of a gas move in straight lines with high average velocity, continually encounter one another and thus change their individual velocities and directions, and cause pressure by their impact against the walls of a container —called also kinetic theory of gases