🚀 go-pugleaf

>What is the name of the distortion which does not appear at the
>harmonic frequencies of the fundamental component but at some
>other frequencies? This type of distortion is often introduced
>by nonlinear filters.
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In a memoryless, non-time-varying analog system, driven by a sinusoid,
you cannot get distortion at frequencies at other than the harmonics
of the fundamental.  this is a direct result of the fact that any
memoryless nonlinear system can be represented by a taylor series expansion.

If the system has memory, you still cannot get signals at frequencies
other than harmonics. This comes from examining volterra series
expansions of such systems. 

If the system is time varying, or is a sampled system, one can get
either "mixing products" or "aliasing", so I guess the answer to your 
question is one of those terms. since this is a DSP group, I guess the
correct term is "aliasing".