halogen(n = noun.substance) - any of five related nonmetallic elements (fluorine or chlorine or bromine or iodine or astatine) that are all monovalent and readily form negative ions;
is a kind of group, grouping
has particulars: astatine, at, atomic number 85, atomic number 35, br, bromine, atomic number 17, chlorine, cl, atomic number 9, f, fluorine, atomic number 53, i, iodin, iodine

   An electro-negative element or radical, which, by combination with a metal, forms a haloid salt; especially, chlorine, fluorine, bromine, and iodine; sometimes, also cyanogen. See Chlorine family, under Chlorine. [1913 Webster]

halogen, n. Chem. any of the group of non-metallic elements: fluorine, chlorine, bromine, iodine, and astatine, which form halides (e.g. sodium chloride) by simple union with a metal.

Derivative:
halogenic adj.

Etymology:
Gk hals halos salt