O III emission refers to the emission of ionized oxygen atoms, specifically the doubly ionized oxygen atom (O III), in astrophysical phenomena. This type of emission is commonly observed in ionized nebulae, such as planetary nebulae and H II regions, as well as in the spectra of active galactic nuclei (AGN) and star-forming galaxies. O III emission lines are often used by astronomers to study the physical conditions of the gas in these environments, such as temperature, density, and chemical composition. By analyzing the intensity and shape of O III emission lines, researchers can gain important insights into the processes and dynamics occurring within these astrophysical objects.