Wiktionary
emission line
n. A spectral line resulting from the emission of electromagnetic radiation from an atom or other species as it decays from an excited state to one of lower energy.
Usage examples of "emission line".
Representations of atoms, the periodic table, quantum states and their changes- The nucleus of hydrogen-1 was a unit of mass, its neutral emission line in space a unit of length, the frequency an inverse unit of time.
Atlan, the sole observer-Atlan, fated to remain unaging for 10,000 years, his life full of inconceivably rich experiences-was staring with eyes torn wide open at this gleaming emission line that visibly grew every five minutes.