The Balmer spectrum of hydrogen.
An emission line is a bright line in the spectrum of a luminous object caused by the emission of light at a particular wavelength. Emission lines may appear on their own, as in the spectrum of a nebula energized by radiation from a nearby hot star, or they may be superimposed on an absorption spectrum, as happens when a star is surrounded by hot gas.
An emission spectrum is a spectrum that consists predominantly or solely of emission lines.
Notable emission lines include the D lines of sodium in the ywllow part of the spectrum.