sheathing

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A layer of boards or of other wood or fiber materials applied to the outer studs, joists, and rafters of a building to strengthen the structure and serve as a base for an exterior weatherproof cladding.
• noun Nautical An exterior covering on the underwater part of a ship's hull that protects it against marine growths.
• noun The act of providing sheathing.

from The Century Dictionary.

• Inclosing by or as by a sheath: as, the sheathing base of a leaf; sheathing stipules, etc. See cut under sheath, 2.
• noun The act of one who sheathes.
• noun That which sheathes, covers, or protects, or may be used for such purpose.
• noun A protection for the main deck of a whaling-vessel, as pine boards, about one inch in thickness, laid over the deck to prevent it from being cut up by the spades, being burned while trying out oil, etc.

from the GNU version of the Collaborative International Dictionary of English.

• Inclosing with a sheath.
• noun The casing or covering of a ship's bottom and sides; the materials for such covering.
• noun (Arch.) The first covering of boards on the outside wall of a frame house or on a timber roof; also, the material used for covering; ceiling boards in general.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Something that wraps around or surrounds something, as a sheath encases its blade.
• verb Present participle of sheathe.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun protective covering consisting, for example, of a layer of boards applied to the studs and joists of a building to strengthen it and serve as a foundation for a weatherproof exterior