Gay-Lussac's law states that the pressure of a gas is directly proportional to its absolute temperature when the volume is held constant. This means that if the temperature of a gas increases, its pressure also increases, provided the volume does not change. This relationship can be expressed mathematically as P \propto T , where P is pressure and T is temperature in Kelvin. Joseph Louis Gay-Lussac, a French chemist and physicist, formulated this law in the early 19th century, contributing significantly to the understanding of gas behavior.