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]

WordNet (r) 3.0 (2006):

luciferin
    n 1: pigment occurring in luminescent organisms (as fireflies);
         emits heatless light when undergoing oxidation