Luciferins are a class of light-emitting biological pigment

Biological pigment

Biological pigments, also known simply as pigments or biochromes are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments...

s found in organisms that cause bioluminescence

Bioluminescence

Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...

. The term is used generically to refer to any light-emitting molecule utilized by a luciferase

Luciferase

Luciferase is a generic term for the class of oxidative enzymes used in bioluminescence and is distinct from a photoprotein. One famous example is the firefly luciferase from the firefly Photinus pyralis. "Firefly luciferase" as a laboratory reagent usually refers to P...

 or photoprotein

Photoprotein

Photoproteins refer to bioluminescent proteins from luminous organisms. These proteins do not exhibit a luciferin-luciferase reaction, that is, a normal enzyme-substrate reaction. Instead, these proteins display luminescence proportional to the amount of the protein...

.
Luciferins are a class of small-molecule substrates

Substrate (biochemistry)

In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...

 that are oxidized

Redox

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 in the presence of the enzyme luciferase

Luciferase

Luciferase is a generic term for the class of oxidative enzymes used in bioluminescence and is distinct from a photoprotein. One famous example is the firefly luciferase from the firefly Photinus pyralis. "Firefly luciferase" as a laboratory reagent usually refers to P...

 to produce oxyluciferin and energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 in the form of light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

. It is not known just how many types of luciferins there are, but some of the better-studied compounds are listed below.