The emission of light by living organisms, such as fireflies, glowworms, and certain fish, jellyfish, plankton, fungi, and bacteria. It occurs when a pigment (usually luciferin) is oxidized without giving off heat. Although it is believed that bioluminescence is involved in animal communication, its function in many organisms has yet to be understood. Bioluminescence is a form of chemiluminescence. Compare chemiluminescence.