2. The oxidized form of any luciferin.
It is produced by enzymic oxidation of a luciferin, the enzyme being a luciferase. This oxididation forms a molecule of oxyluciferin in an electronically excited state, and the molecule can fall to its ground state on emission of a quantum of light.
The wavelength of the light emitted is determined by the nature of both the oxyluciferin and the luciferase to which it is bound.