The emission of absorbed energy (as light) due to a chemical reaction of the components of the system. It includes the subclasses bioluminescence and oxyluminescence, in which light is produced by chemical reactions involving organisms and oxygen, respectively. Chemiluminescence occurs in thousands of chemical reactions covering a wide variety of compounds, both organic and inorganic. Emission of light by fireflies is a common example of bioluminescence.See Luminescence.