A scientific observation by French scientist Joseph Louis Gay-Lussac (1778–1850) where for a gas held at constant volume, there is a direct correlation between temperature and pressure. That is, if you take a gas and put it into a rigid container and double the temperature (as measured from absolute zero), then you double the pressure.
Usually written mathematically as:
P1 / T1 = P2 / T2
The lack of a volume component means that the actual volume of the gas is unimportant as long as it is held constant.
Gay-Lussac's Law is subsumed by the Ideal Gas Law after unification with Boyle's Law and Charles' Law. Like the Ideal Gas Law, Gay-Lussac's Law deviates from reality slightly when measuring real gasses at high accuracy.