In audio, distortion is one of those words that is far more generic in meaning than most people realize when they use it. Technically distortion is ANY deviation in the shape of an audio waveform between two points in a signal path. Given that understanding, just about any audio process (equalization, compression) are all forms of distortion. Some just happen to be (hopefully) good ones. Other kinds of distortion (harmonic distortion, Aliasing, clipping, crossover distortion) are considered undesirable, though at times they are put to good use and considered a good thing.