A "protective sheath" is a covering or layer that shields something from harm or damage, acting like armor or a shield to keep it safe. Full definition
For example, in the southern Weddell Sea so much sea ice forms during the autumn and winter months that the amount of salt released in the process turns the water around and below the 450,000 km2 Filchner - Ronne Ice Shelf into a massive protective sheath. (sciencedaily.com)
These produce protective sheaths that wrap around the axons of peripheral neurons. (sciencedaily.com)
Myelin is the fatty protective sheath that covers every nerve in the body. (laurelwoodvets.com)