luciferin

from WordNet (r) 3.0 (2006)

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

from 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]