The statement defining the relationship between pressure and volume of a gas at constant temperature is Boyle's Law. It states that:

The pressure of a given mass of an ideal gas is inversely proportional to its volume at constant temperature.

This can be expressed mathematically as:

P₁V₁ = P₂V₂

Where:

* P₁ = initial pressure

* V₁ = initial volume

* P₂ = final pressure

* V₂ = final volume

This means that if the pressure of a gas is increased, its volume will decrease proportionally, and vice versa, as long as the temperature remains constant.