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

a general term used for compounds that possess light emitting capabilities

Any of several organic compounds that, in the presence of the enzyme, luciferase, react with oxygen to produce bioluminescent light. Luciferins vary in chemical structure from organism to organism. The luciferin in fireflies, for example, is different from the luciferin in bioluminscent squid. Each type of luciferin is catlyzed by a specific luciferase See also Luciferase. For more detail, see Bioluminescence: Chemical Reaction

compound whose activated form emits light.

Luciferins (from the Latin lucifer, "light-bringing" http://www.bartleby.com/61/99/L0279900.html) are a class of light-emitting biological pigments found in organisms capable of bioluminescence.