A harmonic spectrum is a spectrum containing only frequency components whose frequencies are whole number multiples of the fundamental frequency; such frequencies are known as harmonics. "The individual partials are not heard separately but are blended together by the ear into a single tone."[1]
In other words, if
\omega
\{...,-2\omega,-\omega,0,\omega,2\omega,...\}.
A standard result of Fourier analysis is that a function has a harmonic spectrum if and only if it is periodic.