The term 'airship' generally refers to a dirigible, or blimp. These are aircraft which are kept aloft by the lifting force of a gas, usually helium, which is lighter than air. Such aircraft are very large for the number of people they car carry, and extremely difficult to operate in windy conditions. Most police forces that employ aircraft use helicoptors, and some use fixed wing aircraft.
Some police departments refer to their helicopters as airships, even though they're not truly airships. It's just a nickname.