The total infrared radiation emitted by the Earth and its atmosphere in the temperature range of approximately 200-300K. Because the Earth is nearly a perfect radiator, the radiation from its surface varies as the fourth power of the surface's absolute temperature. Terrestrial radiation provides a major part of the potential energy changes necessary to drive the atmospheric wind system and is responsible for maintaining the surface air temperature within limits for livability.
https://earthobservatory.nasa.gov/glossary/t