air jacket

noun3

1. •
   A lifejacket.

   • AIR-JACKET. A leathern garment furnished with inflated bladders, to buoy the wearer up in the water. — 1867
2. •
   An inflatable vest worn to cushion falls.
3. •
   An enclosed space containing nothing but air that surrounds something in order to provide insulation.

   • In the first Daimler gasoline engines a current of air was forced through an air jacket surrounding the cylinder by means of a rotary fan mounted on the crank shaft. — 1906