(a) Harmonic Factor of $n^{th}$ Harmonic $(HF_n)$: The harmonic factor is a measure of the individual harmonic contribution in the output voltage of an inverter.

It is defined as the ratio of the rms voltage of a particular harmonic component to the rms value of fundamental component.

$$\therefore \quad \mathrm{HF}_{n}=\frac{E_{n_{\mathrm{rms}}}}{E_{1_\mathrm{rms}}}$$

(b) Total Harmonic Distortion (THD): A total harmonic distortion is a measure of closeness in a shape between the output voltage waveform and its fundamental component. It is defined as the ratio of the rms value of its total harmonic component of the output voltage and the rms value of the fundamental component. Mathematically,

$$\mathrm{THD}=\sqrt{\sum_{n=2,3, \ldots}^{\infty} E_{n_{\mathrm{rms}}}^{2}} / E_{1_{\operatorname{rms}}}$$

$$=\sqrt{\frac{E_{0_{\operatorname{rms}}}^{2}-E_{1}^{2}}{E_{1}}}$$

(c) Distortion Factor (DF): A distortion factor indicates the amount of harmonics that remain in the output voltage waveform, after the waveform has been subjected to second-order attenuation (i.e. divided by $n^{2} ) .$ It is defined as

$$\mathrm{DF}=\sqrt{\sum_{n=2,3, \ldots}^{\infty} (\frac{E_{n_{rms}}}{n^2})^2 / E_{1_{\operatorname{rms}}}}$$

(d) Lowest-Order Harmonics (LOH): The lowest frequency harmonic, with a magnitude greater than or equal to three-percent of the magnitude of the fundamental component of the output voltage, is known as lowest-order harmonic. Higher the frequency of the LOH, lower will be the distortion in the current waveform.