At a constant temperature, the pressure of a fixed amount of ideal gas varies inversely with the volume occupied. This relation is known as Boyle’s law. A modern statement of Boyle's law is

The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.

$$ P \propto \frac{1}{V}$$ OR $$PV=k$$

where P is the pressure of the gas, V is the volume of the gas, and k is a constant.