An observation by British scientist Robert Boyle (1627-1691) that when a gas is kept at a constant temperature, there is a direct inverse correlation between the pressure and volume. That is, if you take some gas at a set temperature, and you double the volume the gas occupies, the pressure drops to half the original pressure.
Usually written mathematically as P1V1=P2V2
The lack of a temperature factor demonstrates that the absolute temperature of the gas is not important, as long as it is held constant.
This scientific law was subsumed under the Ideal Gas Law, when it was unified with Charles' Law and Gay-Lussac's Law.
Like the Ideal Gas Law, Boyle's Law deviates from reality slightly when real gasses are measured accurately.