Airglow (also called nightglow) is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark (even after the effects of starlight and diffused sunlight from the far side are removed).
Read more about Airglow: Development, Description, How To Calculate The Effects of Airglow, Induced Airglow, Experimental Observation, Observation of Airglow On Other Solar System Planets