Spectral line

Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning a spectrometer to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 GHz (or a wavelength of 2.7 mm).