The kinetic theory of gases

Gases, liquids or solid phases are composed of an extremely large number of microscopic particles each possessing a certain amount of kinetic energy. This kinetic energy is divided among the particles that comprise the substance.

When referring to a gas, the molecules use this energy in order to move or to rotate. Even though the velocity, rotation or vibration of a single molecule can be measured, it is impossible in practice to directly measure the total kinetic energies of all the particles which make up some macroscopic material like the total amount of gas in a tank.

That's why temperature is typically used to measure or define the average microscopic kinetic energy of the particles in some material.

In ideal gases, the Ideal Gas Law describes the relationship between the pressure and temperature of a gas.