kinetic theory

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A theory concerning the thermodynamic behavior of matter, especially the relationships among pressure, volume, and temperature in gases. It is based on the dependence of temperature on the kinetic energy of the rapidly moving particles of a substance. According to the theory, energy and momentum are conserved in all collisions between particles, and the average behavior of the particles can be deduced by statistical analysis.

WordNet 3.0

1. n. (physics) a theory that gases consist of small particles in random motion