Calculating Total Harmonic Distortion using only the first harmonic
I'm rolling this up into an answer since it is important. Using a 1Hz square wave as an example, the frequency components are 1Hz, 3Hz, 5Hz, 7Hz, 9Hz, etc... all the way up to infinite.

Fundamental: 1Hz, by any definition. The lowest frequency. The period of the waveform.

Harmonic: The technically accurate definition for harmonic is an integer multiple of the fundamental. Therefore, the nth harmonic is n times the fundamental frequency. In the example, 3Hz is the third harmonic and 1Hz is the first harmonic (aka the fundamental).

However, you will find many engineers use "harmonic" when they mean to say overtone.

Overtone: An overtone refers to the most prominent frequency components in the spectrum of a signal in numerical sequence above the and NOT including the fundamental. Therefore, in the example, the first overtone is 3Hz, and the second overtone is 5Hz. 1Hz, the fundamental frequency, is not an overtone at all.

To make thing worse, you can also find engineers using both the technically correct, and commonly understood but incorrect definitions of "harmonic"  where the meaning might changes based on context since we already know what we're talking about. For example, I say square waves are made up of only odd harmonics (technically correct), but then I might absentmindedly say that 3Hz is the first harmonic in in a 1Hz square wave, when I really should be saying first overtone or third harmonic.

For the record, I have never heard "overtone" used in any engineering class of mine. Ever. The only reason I know of it is from music classes. It needs to be a thing because those musicians have been talking about frequencies far longer than we have. They already figured it out.