What does "helmet" mean?

Additional senses

1. 2.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.
2. 3.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) (Zoöl.) A naked shield or protuberance on the top or fore part of the head of a bird. Helmet beetle (Zoöl.), a leaf-eating beetle of the family Chrysomelidæ, having a short, broad, and flattened body. Many species are known. -- Helmet shell (Zoöl.), 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 (Zoöl.), an African wood shrike of the genus Prionodon, having a large crest.