Spectra generated by excited substances. Consists of radiation with only specific wavelengths, corresponding to transitions between states in free atoms.
Example: The line spectrum of hydrogen contains 4 sharp lines in the visible part of the spectrum.
A line spectrum is a spectrum where the energy is concentrated at specific frequencies (lines or bins), as opposed to a continuous spectrum where the energy is smeared out over a band of frequencies.
A deterministic signal will have a line spectrum, and a random signal will have a continuous spectrum.
Spectra generated by machine vibration signatures are always a combination of these two types.