A sound that is not part of the original signal. Distortion can be a modification of the original signal (intermodulation distortion), or generating new signals that result from the interference of a combination of signals (harmonic distortion). Can be characterized as a roughness, fuzziness, harshness, or stridency in the music. There are many distortions that can be measured: IM (intermodulation distortion), harmonic distortion, and TIM (transient intermediation distortion).
Distortion can cause breakup, where the headphone can’t handle the power or the low frequencies, which causes sound to crack up.