Luciferin is a pigment protein made by organisms which are capable of bioluminescence, such as fireflies and some species of deep-sea fish. The pigment combines with ATP to form luciferyl adenylate, which is then combined with oxygen by the enzyme luciferase to make faint visible light.
This pigment was named after Lucifer, former Angel of Light.
This writeup is partly based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/