Boyle's Law
Definition
At constant temperature, squeezing a gas into a smaller volume raises its pressure: pressure and volume are inversely related.
Technical definition
Boyle's law states that for a fixed amount of gas at constant temperature, pressure and volume are inversely proportional. It is the constant-temperature special case of the ideal gas law.
Example
Pushing a sealed syringe from 60 mL to 30 mL at constant temperature doubles the gas pressure inside.
Common confusion
Boyle's law only holds at constant temperature AND constant amount of gas. Let gas leak out or change the temperature and the simple inverse relationship breaks.