One definition of distortion would be to say any deviation whatsoever of the original audio signal. If the output of our amplifier is identical in every respect to its input then we can say its undistorted.
But what if we roll off the bottom or top end yet leave everything else intact? Is that distortion? After all, the output differs from the input.
It's a matter of viewpoint.
Scientific vs. purist.
Literal vs. realistic.
In the realm of audio engineering, a bandwidth-limited audio signal is not considered distorted if the frequencies within that limited bandwidth remain untouched. Instead of distortion we would refer to this as filtering or attenuation.
Distortion, by the common engineering definition, refers to an alteration of the waveform that adds or removes frequency components within the specified range. So, if the frequencies that do remain within the bandwidth are unaltered, the term "distortion" typically wouldn't apply.
It's all a matter of viewpoint.
