The spectral linewidth characterizes the width of a spectral line, such as in the electromagnetic emission spectrum of an atom, or the frequency spectrum of an acoustic or electronic system. For example, the emission of an atom usually has a very thin spectral linewidth, as only transitions between discrete energy levels are allowed, leading to emission of photons with a certain energy.

Die Linienbreite ist die Breite des von schmalbandigem Licht (z. B. Spektrallinie oder Laserlicht) umfassten Frequenz- bzw. Wellenlängenintervalls.