The theory that bulk matter, to outward appearances motionless, is composed of huge numbers of atoms and molecules in rapid and incessant motion. By applying the kinetic theory of gases, relations between some of the bulk properties of gases (e.g., the ideal gas law) may be derived and given a molecular interpretation.
Kinetic theory attempts to explain macroscopic properties of gases, such as pressure, temperature, or volume, by considering their molecular composition and motion. Essentially, the theory posits that pressure is due not to static repulsion between molecules, as was Isaac Newton's conjecture, but due to collisions between molecules moving about with a certain velocity. Kinetic theory is also known as kinetic-molecular theory or collision theory.