The Collaborative International Dictionary of English v.0.48:

Sodium \So"di*um\, n. [NL., fr.E. soda.] (Chem.)
   A common metallic element of the alkali group, in nature
   always occuring combined, as in common salt, in albite, etc.
   It is isolated as a soft, waxy, white, unstable metal, so
   highly reactive that it combines violently with water, and to
   be preserved must be kept under petroleum or some similar
   liquid. Sodium is used combined in many salts, in the free
   state as a reducer, and as a means of obtaining other metals
   (as magnesium and aluminium) is an important commercial
   product. Symbol Na (Natrium). Atomic weight 22.990.
   Specific gravity 0.97.
   [1913 Webster]

   Sodium amalgam, an alloy of sodium and mercury, usually
      produced as a gray metallic crystalline substance, which
      is used as a reducing agent, and otherwise.

   Sodium carbonate, a white crystalline substance,
      Na2CO3.10H2O, having a cooling alkaline taste, found in
      the ashes of many plants, and produced artifically in
      large quantities from common salt. It is used in making
      soap, glass, paper, etc., and as alkaline agent in many
      chemical industries. Called also sal soda, washing
      soda, or soda. Cf. Sodium bicarbonate, and Trona.


   Sodium chloride, common, or table, salt, NaCl.

   Sodium hydroxide, a white opaque brittle solid, NaOH,
      having a fibrous structure, produced by the action of
      quicklime, or of calcium hydrate (milk of lime), on sodium
      carbonate. It is a strong alkali, and is used in the
      manufacture of soap, in making wood pulp for paper, etc.
      Called also sodium hydrate, and caustic soda. By
      extension, a solution of sodium hydroxide.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sodium hydroxide
    n 1: a strongly alkaline caustic used in manufacturing soap and
         paper and aluminum and various sodium compounds [syn:
         sodium hydroxide, caustic soda]