Source : Webster's Revised Unabridged Dictionary (1913)

Firefly \Fire"fly`\, n.; pl. {Fireflies}. (Zo["o]l.)
   Any luminous winged insect, esp. luminous beetles of the
   family {Lampyrid[ae]}.

   Note: The common American species belong to the genera
         {Photinus} and {Photuris}, in which both sexes are
         winged. The name is also applied to luminous species of
         {Elaterid[ae]}. See {Fire beetle}.

Source : WordNet®

firefly
     n 1: tropical American click beetle having bright luminous spots
          [syn: {fire beetle}, {Pyrophorus noctiluca}]
     2: nocturnal beetle common in warm regions having luminescent
        abdominal organs [syn: {lightning bug}]