An airtight protection mask to be fitted over the face, usually made of rubber with plastic or glass windows for eyes, straps to make it stay on place, and a filter through which all breathing air goes.
The filters typically contain charcoal to neutralize some poisonous gases.
Used commonly by people who get exposed to poisonous/irritating gases in their jobs - most notably by soldiers.
Of course, gas masks in themselves are not enough of a protection in most cases. They are often very effective against "irritating" gases (like tear gases), but not much for anything actually lethal (like nerve gases). | 
