Harmonic Distortion
Definition
Distortion that adds harmonic overtones — multiples of the original frequency — to an audio signal. Even-order harmonics (2nd, 4th) are typically perceived as warm and musical; odd-order harmonics (3rd, 5th) can sound harsh and abrasive at higher amounts.
In Simple Terms
Extra tones added to a sound by equipment or plugins — some warm and pleasant (even harmonics from tubes), some harsh and aggressive (odd harmonics from transistors). It's the secret ingredient behind that "analog warmth" people love.
In Practice
A tube preamp driven gently adds predominantly 2nd-order harmonic distortion to a vocal, giving it warmth and presence. A transistor circuit clipping hard adds odd-order harmonics, which sound harder and more aggressive.