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