bioluminescence
Light produced by chemical reactions inside living organisms. It occurs in a diverse array of living species, including fungi, microscopic plankton, jellyfish, squid, and anglerfish.
Fireflies are likely the most familiar terrestrial example of bioluminescence in nature. In anime art, they are ubiquitous in outdoor environments during summer nights. To avoid crowding out all the less common uses of this tag, fireflies shouldn't be tagged bioluminescence.
See also
This tag implicates glowing (learn more).