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]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "firefly":
   blink, blinking, candle fly, coruscation, fire beetle, fireworm,
   glimmer, glimmering, glisten, glister, glitter, glittering,
   glowworm, lampyrid, lantern fly, lightning bug, luciferase,
   luciferin, luminophor, phosphor, scintilla, scintillation, shimmer,
   shimmering, spangle, spark, sparkle, stroboscopic light, tinsel,
   twinkle, twinkling