Bioluminescence is light emitted by a living organism. There's something magical about the bioluminescence of a valley full of fireflies on a summer night.
Fireflies may be the most familiar example of bioluminescence, but plenty of other animals, plants, fungi, and bacteria also have the ability to generate a glowing light. Most of these organisms live in the deepest part of the ocean, including jellyfish, about half of which use bioluminescence to defend themselves against predators. The bio- part of the word means "living thing," and luminescence is from the Latin root lumen, or "light."