Wordnet 3.0
NOUN (1)
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]
WordNet (r) 3.0 (2006):
atomic number 17
    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]