Sheath

n. case, cover (for a dagger, sword, etc.); tubular part or structure which closely surrounds or envelopes (Biology); close-fitting dress

The word sheath has a number of related meanings in English. In general usage, a sheath is any protective covering that fits closely around the object to be protected. For short-bladed weapons such as knives, dirks, and daggers, protection is provided by a sheath and retaining strap. For longer weapons such as swords and sabres, the term scabbard is preferred.
See more at Wikipedia.org...

Sheath a tubular case or envelope. arachnoid sheath the continuation of the arachnoidea mater around the optic nerve, forming part of its internal [more]Sheath - Community and Resources

Noun
1. a protective covering for a knife or sword
(hypernym) protective covering, protective cover, protection
(hyponym) aglet, aiglet
2. an enveloping structure or covering enclosing an animal or plant organ or part
(synonym) case
(hypernym) covering, natural covering, cover
(hyponym) theca
3. a dress suitable for formal occasions
(synonym) cocktail dress
(hypernym) dress, frock

(n.)
The base of a leaf when sheathing or investing a stem or branch, as in grasses.
  

(n.)
One of the elytra of an insect.
  

(n.)
Any sheathlike covering, organ, or part.
  

(n.)
A case for the reception of a sword, hunting knife, or other long and slender instrument; a scabbard.