depressor

A degressor is a type of instrument that decreases air pressure.

When we talk about a pressurized aircraft cabin, people might refer to a degressor as part of its safety system. A degressor helps reduce the pressure inside the plane when it's losing altitude quickly. It's usually something you don't notice unless there's an emergency on board – then pilots use the degressor to get everyone to breathe easier in case the plane needs to make an emergency landing.