The Collaborative International Dictionary of English v.0.48:

Luciferin \Lu*cif"er*in\ (l[=oo]*s[i^]f"[~e]r*[i^]n), n.
   (Biochem.)
   any of several substances found in luminescent organisms
   (such as the firefly, Photinus pyralis) which, when
   oxidized, produces an almost heatless light. It was first
   isolated from fireflies, and is the source of the firefly
   luminescence. See also firefly luciferin
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Firefly luciferin \Fire"fly` lu*cif"er*in\, n. (Biochem.)
   a type of luciferin produced by the firefly Photinus
   pyralis.

   Note: Its structure has been elucidated, and chemically it is
         recognized as

         4,5-Dihydro-2-(6-hydroxy-2-benzothiazol-2-yl)-4-thiazolecarboxylic
         acid, C11H8N3O2S2. It has found use in a very
         sensitive assay for ATP, in which concentrations of ATP
         as low as 10^-11 molar can be detected. --[MI11]
         [PJC]

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]