The Collaborative International Dictionary of English v.0.48:

Firefly \Fire"fly`\, n.; pl. Fireflies. (Zool.)
   Any luminous winged insect, esp. luminous beetles of the
   family Lampyrid[ae].
   [1913 Webster]

   Note: The common American species belong to the genera
         Photinus (especially Photinus pyralis) and
         Photuris, in which both sexes are winged. The name is
         also applied to luminous species of Elaterid[ae]. See
         Fire beetle.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

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