sheath / shē[unvoicedth]/ • n. (pl. sheaths / shē[voicedth]z; shē[unvoicedth]s/ ) a close-fitting cover for something, esp. something that is elongated in shape, in particular: ∎  a cover for the blade of a knife or sword. ∎  a structure in living tissue that closely envelops another: the fatty sheath around nerve fibers. ∎  (also sheath dress) a woman's close-fitting dress: a tight sheath of black and gold lurex. ∎  a protective covering around an electric cable. ∎  a condom.DERIVATIVES: sheath·less adj.

sheath
1. Of leaves, the base of a blade or stalk that encloses the stem.

2. (of cyanobacteria) See CAPSULE (of prokaryotes).

3. See FUNGAL SHEATH.

sheath OE. sċæð, sċeað = OS. skēðia (Du. scheede, schee), OHG. sceida (G. scheide), ON. skeiðir pl. scabbard :- Gmc. *skaiþiz, *skaiþjō, prob. f. *skaiþ- divide (see SHED1).

sheath (sheeth) n. (in anatomy) the layer of connective tissue that envelops structures such as nerves, arteries, tendons, and muscles.