Harmonic distortion occurs when a signal is altered, creating additional frequencies that are multiples of the original frequency. This can happen in various systems, such as audio equipment or electrical circuits, and can lead to a change in sound quality or signal integrity. The unwanted harmonics can interfere with the desired signal, making it less clear or accurate. In audio systems, harmonic distortion is often measured as a percentage, indicating how much of the output signal is distorted compared to the original input. High levels of distortion can result in a harsh or unpleasant sound, while lower levels are generally more desirable for maintaining audio fidelity.