The lowest possible temperature. It is the point where particles reach their minimum of vibrational motion. For instance, the temperature of a gas is proportional to the average kinetic energy of the moving molecules or atoms. In classical terms, absolute zero would be reached when the gas particles do not move anymore. According to quantum mechanics the particles still retain a minimum motion, induced by lowest possible energy state that a quantum mechanical system may have and which fluctuates according to the Heisenberg uncertainty principle.