The Collaborative International Dictionary of English v.0.48:

Chlorine \Chlo"rine\, n. [Gr. ? pale green, greenish yellow. So
   named from its color. See Yellow.] (Chem.)
   One of the elementary substances, commonly isolated as a
   greenish yellow gas, two and one half times as heavy as air,
   of an intensely disagreeable suffocating odor, and
   exceedingly poisonous. It is abundant in nature, the most
   important compound being common salt (Sodium chloride). It is
   powerful oxidizing, bleaching, and disinfecting agent. Symbol
   Cl. Atomic weight, 35.4.
   [1913 Webster]

   Chlorine family, the elements fluorine, chlorine, bromine,
      and iodine, called the halogens, and classed together
      from their common peculiarities.
      [1913 Webster]

WordNet (r) 3.0 (2006):

chlorine
    n 1: a common nonmetallic element belonging to the halogens;
         best known as a heavy yellow irritating toxic gas; used to
         purify water and as a bleaching agent and disinfectant;
         occurs naturally only as a salt (as in sea water) [syn:
         chlorine, Cl, atomic number 17]

The Elements (07Nov00):

chlorine
Symbol: Cl
Atomic number: 17
Atomic weight: 35.453
Halogen element. Poisonous greenish-yellow gas. Occurs widely in nature
as
sodium chloride in seawater. Reacts directly with many elements and
compounds, strong oxidizing agent. Discovered by Karl Scheele in 1774.
Humphrey David confirmed it as an element in 1810.