1. A substance found in the cells of bioluminescent organisms; such as, glowworms and fireflies, that on oxidation emits bluish-green light with very little heat.
2. Any of several substances involved in the emission of light by a living organism.

Luciferins are chemically diverse, depending on the organism. They are oxidized enzymically (with luciferase) by molecular oxygen to form a product in an electronically excited state. This product can lose its excitation with emission of light.