sheath

Sheath refers to a protective covering that encloses and protects another object. The form and function of a sheath vary by context, from mechanical protections to biological envelopes.

In everyday use, a sheath is a case for a blade or dagger, often called a scabbard. It keeps a blade safe when not in use and allows easy access, and is typically made of leather, wood, metal, or plastic and worn at the belt or on the leg.

In engineering and telecommunications, the sheath is the outer layer of a cable or wire, providing insulation, protection, and environmental sealing. Materials include plastics and rubber, and the sheath can shield against abrasion, moisture, and chemicals.

In biology and medicine, sheath denotes a surrounding layer of tissue. A tendon sheath is a synovial membrane that envelops a tendon to reduce friction during movement. A nerve sheath refers to the protective coverings around a nerve, comprising connective tissue layers such as endoneurium, perineurium, and epineurium. In plants, the leaf sheath is the basal, tubular part of a grass leaf that clasps the stem.

In broader usage, the term can describe an enclosing boundary or protective layer in other contexts, emphasizing the common idea of a surrounding, safeguarding envelope that preserves structure and function.