kinetic theory

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A theory of the thermodynamic behavior of matter, especially the relationships among pressure, volume, and temperature in gases, based on the dependence of temperature on the kinetic energy of the rapidly moving particles of a substance. The theory uses statistical mechanics under the presumption that energy and momentum are conserved in all collisions between particles.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (physics) a theory that gases consist of small particles in random motion