Sheath

Sheath: Of a communications or power cable, the outer covering or coverings of tough material, often plastic, that is resistant to environmental hazards such as abrasion, liquid intrusion, solar radiation, etc., and is used to protect cable component(s) such as optical fibers or metallic conductors that transport the signal or power.

Note: There may be more than one sheath surrounding a given cable. For example, some cable designs use an inner sheath surrounded by metallic armor, over which is an outer sheath. Synonym jacket.

Source: from Federal Standard 1037C