From AMS Glossary
Dark absorption lines observed in a spectrum of solar radiation obtained with an instrument located near the earth's surface.
These lines are produced by constituents of the atmosphere of the earth itself. They appear relatively narrow due to the cool temperature of the atmospheric gases, and are quite distinct from the broad Fraunhofer lines caused by absorption by high-temperature solar gases. The terrestrial nature of these narrow telluric lines is revealed by their intensity variation with solar zenith angle and by their freedom from any Doppler broadening due to solar rotation. Water vapor produces the strongest of the telluric lines in the visible spectrum.