The Collaborative International Dictionary of English v.0.48:

Fluorine \Flu"or*ine\ (fl[=u]"[o^]r*[i^]n or fl[=u]"[o^]r*[=e]n;
   104), n. [NL. fluorina: cf. G. fluorin, F. fluorine. So
   called from its occurrence in the mineral fluorite.] (Chem.)
   A non-metallic, gaseous element of atomic number 9, strongly
   acid or negative, and associated with chlorine, bromine, and
   iodine, in the halogen group of which it is the first member.
   It always occurs combined, is very active chemically, and
   possesses such an avidity for most elements, and silicon
   especially, that it can neither be prepared nor kept in glass
   vessels, but may be contained in lead vessels. If set free it
   immediately attacks a containing glass vessel, so that it was
   not isolated until 1886. It is a pungent, corrosive,
   colorless gas. Symbol F. Atomic weight 19.00.
   [1913 Webster]

   Note: Fluorine unites with hydrogen to form hydrofluoric
         acid, which is the agent employed in etching glass. It
         occurs naturally, principally combined as calcium
         fluoride in fluorite, and as a double fluoride of
         aluminium and sodium in cryolite.
         [1913 Webster]

WordNet (r) 3.0 (2006):

fluorine
    n 1: a nonmetallic univalent element belonging to the halogens;
         usually a yellow irritating toxic flammable gas; a powerful
         oxidizing agent; recovered from fluorite or cryolite or
         fluorapatite [syn: fluorine, F, atomic number 9]

The Elements (07Nov00):

fluorine
Symbol: F
Atomic number: 9
Atomic weight: 18.9984
A poisonous pale yellow gaseous element belonging to group 17 of the
periodic table (The halogens). It is the most chemically reactive and
electronegative element. It is highly dangerous, causing severe chemical
burns on contact with flesh. Fluorine was identified by Scheele in 1771
and first isolated by Moissan in 1886.