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.
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.