Kinetic Molecular Theory
Definition
The model explaining gas behavior by picturing gases as crowds of tiny particles zooming around and colliding elastically, with temperature measuring their average kinetic energy.
Technical definition
Kinetic molecular theory models a gas as a large number of small particles in constant random motion with negligible volume and no intermolecular forces except during elastic collisions. It derives that absolute temperature is proportional to average kinetic energy.
Example
It explains why a hot gas exerts more pressure: faster particles hit the walls harder and more often. It also predicts Graham's law of effusion.
Common confusion
Temperature measures AVERAGE kinetic energy, not the speed of any single particle. At any moment individual molecules move at a wide spread of speeds.