The law that states that the volume of a gas and its pressure are inversely proportional is Boyle's Law.

Here's what it means:

* Inversely Proportional: This means that as one quantity increases, the other decreases at a proportional rate.

* Boyle's Law: At a constant temperature, the volume of a given mass of an ideal gas is inversely proportional to its absolute pressure.

Mathematically:

* P₁V₁ = P₂V₂

Where:

* P₁ = Initial pressure

* V₁ = Initial volume

* P₂ = Final pressure

* V₂ = Final volume