Partials become harmonics only when integer-related to the fundamental

Evidence

The individual sine wave components of the waveform are often called the partials of the waveform. If they are integer related, as in the square wave, then they can be called harmonics. The lowest frequency is called the fundamental, or first harmonic, and the higher-frequency harmonics are labeled according to their frequency multiple relative to the fundamental. Thus the second harmonic is twice the frequency of the fundamental and so on. Partials, on the other hand, need not be harmonically related to the fundamental and are numbered in their order of appearance with frequency. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 450

Categorization

• DEFINE: Clarifies the precise distinction between partials and harmonics — a terminology difference beginners routinely blur.
• XREF: connects to: Spectrum components are partials and only integer-related ones are harmonics — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 450