spectral line

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An isolated bright or dark line in a spectrum produced by emission or absorption of light of a single wavelength.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics a dark or bright line in an otherwise continuous spectrum caused by the absorption or emission of light, at a single wavelength, by the movement of electrons between energy levels of an atom or molecule