Intermodulation distortion (IMD)
New tones created when two frequencies pass through the same nonlinear circuit and mix into sums and differences that were never in the source.
Feed an amplifier 1,000 Hz and 1,100 Hz at once and you get more than the overtones of each, which is what THD measures. You also get 100 Hz, 2,100 Hz, 900 Hz and 1,200 Hz – products of the form f1 ± f2 and 2f1 − f2. None of them sit in a harmonic relationship to the music, so they sound harsher than harmonic distortion at the same level. Two test methods dominate: the SMPTE test with 60 Hz and 7 kHz mixed 4:1, and the CCIF twin-tone test at 19 and 20 kHz, which drops a 1 kHz difference tone right into the audible band.
IMD is also the most concrete argument against ultrasonic content in consumer formats. Nobody hears anything above 20 kHz, but it can intermodulate in amplifiers and speakers and leave audible residue further down – meaning hi-res audio can sound worse, not better.