The law stating that PV^γ remains constant for an ideal gas in a reversible adiabatic process is called Poisson's law. It is also known as Poisson's equation, Poisson's relation, or the adiabatic law.
Here, P is pressure, V is volume, and γ is the heat-capacity ratio, defined as Cp/Cv. The relationship applies when no heat enters or leaves the gas and the process is sufficiently slow and reversible. As the gas expands, it cools and its pressure falls more rapidly than it would during an isothermal expansion.
The same result can be written as TV^(γ−1) = constant or T^γP^(1−γ) = constant. These forms are useful when temperature is known instead of volume. For a monatomic ideal gas, γ is about 5/3; for air, it is commonly approximated as 1.4.
The relation is associated with Siméon Denis Poisson, although Pierre-Simon Laplace also made major contributions to the adiabatic theory of gases and sound. A common mix-up is Boyle's law, PV = constant, which applies to an isothermal rather than an adiabatic process.