The Collaborative International Dictionary of English v.0.48:

Hood \Hood\, n. [OE. hood, hod, AS. h[=o]d; akin to D. hoed hat,
   G. hut, OHG. huot, also to E. hat, and prob. to E. heed.
   [root]13.]
   1. State; condition. [Obs.]
      [1913 Webster]

            How could thou ween, through that disguised hood
            To hide thy state from being understood? --Spenser.
      [1913 Webster]

   2. A covering or garment for the head or the head and
      shoulders, often attached to the body garment; especially:
      (a) A soft covering for the head, worn by women, which
          leaves only the face exposed.
      (b) A part of a monk's outer garment, with which he covers
          his head; a cowl. "All hoods make not monks." --Shak.
      (c) A like appendage to a cloak or loose overcoat, that
          may be drawn up over the head at pleasure.
      (d) An ornamental fold at the back of an academic gown or
          ecclesiastical vestment; as, a master's hood.
      (e) A covering for a horse's head.
      (f) (Falconry) A covering for a hawk's head and eyes. See
          Illust. of Falcon.
          [1913 Webster]

   3. Anything resembling a hood in form or use; as:
      (a) The top or head of a carriage.
      (b) A chimney top, often contrived to secure a constant
          draught by turning with the wind.
      (c) A projecting cover above a hearth, forming the upper
          part of the fireplace, and confining the smoke to the
          flue.
      (d) The top of a pump.
      (e) (Ord.) A covering for a mortar.
      (f) (Bot.) The hood-shaped upper petal of some flowers, as
          of monkshood; -- called also helmet. --Gray.
      (g) (Naut.) A covering or porch for a companion hatch.
          [1913 Webster]

   4. (Shipbuilding) The endmost plank of a strake which reaches
      the stem or stern.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Helmet \Hel"met\, n. [OF. helmet, a dim of helme, F. heaume; of
   Teutonic origin; cf. G. helm, akin to AS. & OS. helm, D.
   helm, helmet, Icel. hj[=a]lmr, Sw. hjelm, Dan. hielm, Goth.
   hilms; and prob. from the root of AS. helan to hide, to hele;
   cf. also Lith. szalmas, Russ. shleme, Skr. [,c]arman
   protection. [root]17. Cf. Hele, Hell, Helm a helmet.]
   1. (Armor) A defensive covering for the head. See Casque,
      Headpiece, Morion, Sallet, and Illust. of Beaver.
      [1913 Webster]

   2. (Her.) The representation of a helmet over shields or
      coats of arms, denoting gradations of rank by
      modifications of form.
      [1913 Webster]

   3. A helmet-shaped hat, made of cork, felt, metal, or other
      suitable material, worn as part of the uniform of
      soldiers, firemen, etc., also worn in hot countries as a
      protection from the heat of the sun.
      [1913 Webster]

   4. That which resembles a helmet in form, position, etc.; as:
      (a) (Chem.) The upper part of a retort. --Boyle.
      (b) (Bot.) The hood-formed upper sepal or petal of some
          flowers, as of the monkshood or the snapdragon.
      (c) (Zool.) A naked shield or protuberance on the top or
          fore part of the head of a bird.
          [1913 Webster]

   Helmet beetle (Zool.), a leaf-eating beetle of the family
      Chrysomelid[ae], having a short, broad, and flattened
      body. Many species are known.

   Helmet shell (Zool.), one of many species of tropical
      marine univalve shells belonging to Cassis and allied
      genera. Many of them are large and handsome; several are
      used for cutting as cameos, and hence are called cameo
      shells. See King conch.

   Helmet shrike (Zool.), an African wood shrike of the genus
      Prionodon, having a large crest.
      [1913 Webster]

WordNet (r) 3.0 (2006):

helmet
    n 1: armor plate that protects the head
    2: a protective headgear made of hard material to resist blows

Moby Thesaurus II by Grady Ward, 1.0:

162 Moby Thesaurus words for "helmet":
   achievement, aegis, alerion, animal charge, annulet, argent,
   arm guard, armorial bearings, armory, arms, azure, backstop,
   bandeau, bar, bar sinister, baton, bearings, bend, bend sinister,
   billet, blazon, blazonry, bordure, broad arrow, buffer, bulwark,
   bumper, cadency mark, canton, chaplet, charge, chevron, chief,
   coat of arms, cockatrice, contraceptive, copyright, coronet,
   crash helmet, crescent, crest, cross, cross moline, crown, cushion,
   dashboard, device, difference, differencing, dodger, eagle, ermine,
   ermines, erminites, erminois, escutcheon, face mask, falcon,
   fender, fess, fess point, field, file, finger guard, flanch,
   fleur-de-lis, foot guard, fret, fur, fuse, fusil, garland, goggles,
   governor, griffin, guard, guardrail, gules, gyron, hand guard,
   handrail, hard hat, hatchment, heraldic device, honor point,
   impalement, impaling, inescutcheon, insulation, interlock,
   knee guard, knuckle guard, label, laminated glass, life preserver,
   lifeline, lightning conductor, lightning rod, lion, lozenge,
   mantling, marshaling, martlet, mascle, mask, metal, motto,
   mudguard, mullet, nombril point, nose guard, octofoil, or,
   ordinary, orle, pad, padding, pale, palladium, paly, patent, pean,
   pheon, pilot, preventive, prophylactic, protective clothing,
   protective umbrella, purpure, quarter, quartering, rose, sable,
   safeguard, safety, safety glass, safety plug, safety rail,
   safety shoes, safety switch, safety valve, saltire, screen,
   scutcheon, seat belt, shield, shin guard, spread eagle,
   subordinary, sun helmet, tenne, tincture, torse, tressure,
   umbrella, unicorn, vair, vert, windscreen, windshield, wreath,
   yale



Easton's 1897 Bible Dictionary:

Helmet
   (Heb. kob'a), a cap for the defence of the head (1 Sam. 17:5,
   38). In the New Testament the Greek equivalent is used (Eph.
   6:17; 1 Thess. 5:8). (See ARMS.)