When we run our standard tests for a circuit we send a single frequency tone through it, ignore the output of that tone, and look to see what shouldn't be there. Whatever comes out is called distortion.
In a perfect circuit, adding a 1kHz tone produces nothing more than 1kHz. But, there's no such thing as perfect and so, when that tone is reproduced, there are other tones that appear—multiples of that original frequency called harmonics (which is why the outcome of this is call Harmonic Distortion and when we total up all those unwanted harmonics we get Total Harmonic Distortion or the infamous acronym THD).
If we're feeling particularly evil and want to torture the circuit a little more we step up the game and put two frequency tones through it: our original 1kHz tone, and then another at, say, 7kHz. Now our little circuit under test is challenged to deal with two disparate tones and whatever it produces that is not those original tones is also called distortion, though this time it has a new name. Intermodulation distortion, or IM distortion for short.
And, if we're in a completely sadistic mood, we run a multitone test where dozens upon dozens of frequencies bombard our poor little circuit to see what comes out the other end.
Oh, the cruelty!
But here's the thing. With all this abusive torture going on we shouldn't feel sorry for our circuit. No, we don't grieve because in the real world, our circuit has to deal with far worse—far more complexity—for its day job. The greatest collection of torture tones imaginable is…
Wait for it…
Music.
