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]

The Collaborative International Dictionary of English v.0.48:

Natrium \Na"tri*um\, n. [NL. See Natron.] (Chem.)
   The technical name for sodium.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sodium
    n 1: a silvery soft waxy metallic element of the alkali metal
         group; occurs abundantly in natural compounds (especially
         in salt water); burns with a yellow flame and reacts
         violently in water; occurs in sea water and in the mineral
         halite (rock salt) [syn: sodium, Na, atomic number
         11]

The Elements (07Nov00):

sodium
Symbol: Na
Atomic number: 11
Atomic weight: 22.9898
Soft silvery reactive element belonging to group 1 of the periodic table
(alkali metals). It is highly reactive, oxidizing in air and reacting
violently with water, forcing it to be kept under oil. It was first
isolated by Humphrey Davy in 1807.