Bioluminescence is the ability of a living organism to emit light as the result of a chemical reaction. This ability allows an organism to convert chemical energy into light energy. The term is a hybrid word that originates from the Greek word "bios", which means "living" and the Latin word "lumen", which means "light". Bioluminescence may be generated by symbiotic organisms carried within a larger organism. This can be exhibited by animals as well as some plants and fungi.

Bioluminescence is a form of luminescence that emits cold light and not through thermal radiation. This phenomena is different from phosphorescence, fluorescence and the refraction of light. About ninety percent of marine life found in very deep seas has been known to produce bioluminescence in one form or another.

Most marine bioluminescence emission belongs in the blue and green light spectrum. It is at these wavelengths that light can be transmitted through the seawater most easily. However, there are also other organisms such as certain loose jawed fish that emit red and infrared light. There are also some marine bacteria that can generate bioluminescence which can create "sparkles" in the sea. 

There are also some non-marine organisms that can generate bioluminescence although they are less widely distributed. The two most familiar forms of land organisms that exhibit bioluminescence are the common fireflies as well as New Zealand glow worms. Another example is the railroad worm which is known to be one of the few organisms that can emit light in two different colors. There are also some insect larvae, annelids, arachnids and even species of fungi have also been noted to possess bioluminescent abilities.

Scientists studying the phenomena of bioluminescence have found that there are several reasons as to how different organisms make use of this ability. One of the main reasons is illumination. In the deepest portion of the sea, light from the sun is unable to penetrate and provide illumination for the inhabitants down there.

That is why some deep sea marine life have been able to develop bioluminescence in order to provide some light in an otherwise dark environment. such illumination allows also some marine organisms to better see their prey. The Black Dragon fish, for example, produces a red glow that allows it to see red-pigmented prey, which are normally invisible in the deep ocean environment where red light has been filtered out by the depth of the water.

Bioluminescence is also used by some animals as a lure in order to attract prey. There are some deep sea fishes such as the anglerfish that make use of a dangling appendage extending from the head just for this purpose. This appendage usually has a bioluminescent tip that is used to attract small animals to come within striking distance of the fish.

Some organisms also make use of bioluminescence in order to attract mates. Fireflies make use of periodic flashing in their abdomens in order to attract potential mates during the mating season. Some experts have theorized that animals make use of pheromones for long-distance communication and attraction of mates while bioluminescence is used for potential mates that is within close range.

Some organisms make use of bioluminescence as a protective mechanism. There are certain types of squids and small crustaceans that make use of bioluminescent chemical mixtures or bioluminescent bacterial slurries that can be dispersed in a cloud of scattered luminescent particles. The sudden burst of scattered luminescent particles can confuse or repel a potential predator, giving the squid or crustacean enough time to make an escape. Every species of firefly also has larvae that glow in order to repel predators.