1. A close fitting cover of something; especially, anything that is elongated in shape or structure: One kind of sheath is a safe place to keep the blade of a sword or a knife.
2. The connective tissues in the body that closely cover an organ or parts of the anatomy: There are sheaths around nerve fibers, vessels, muscles, tendons, and joints in bodies.
3. A protective cover of objects: The electrician made sure that the wires were covered with plastic sheaths.
4. Etymology: from Old English scaeth, sceath, "scabbard" of Germanic origin; related to Dutch schede and German Scheide, "split, divide".