Milky seas, also called mareel, is a luminous phenomenon in the ocean in which large areas of seawater appear to glow translucently. Such occurrences glow brightly enough at night to be visible from satellites orbiting Earth.

Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some bioluminescent bacteria, and terrestrial arthropods such as fireflies. In some animals, the light is bacteriogenic, produced by symbiotic bacteria such as those from the genus Vibrio; in others, it is autogenic, produced by the animals themselves.