When referring to distortion components, the second harmonic is at a frequency that is twice the frequency of the fundamental. The third harmonic is at a frequency that is three times the fundamental, the 4th=4X, the 5th=5X, etc.

As an example, using a sine wave of 40kHz as our input signal, our fundamental is 40kHz, the 2nd harmonic is at 80kHz, the 3rd is at 120kHz, the 4th is at 160kHz, etc.

The 2nd, 4th, etc, harmonics are referred to as "even" harmonics and the 3rd, 5th, etc, harmonics are referred to as "odd" harmonics.

The expression "1st harmonic" is not typically used.