Linear vs Non linear distortion
The Wikipedia article on "distortion" is actually pretty good. https://en.wikipedia.org/wiki/Distortion

Partially it's about semantics and how you exactly define the term. By most definition "linear distortion" is used equivalent to "non ideal frequency response" where magnitude and/or phase are a non-trivial function of frequency. 

Non-linear distortion is caused by any type of mechanism that violates linearity. Typical examples are limiting or clipping or compression. 

An LTI does not cause any type of non-linear distortion, however they typically do cause linear distortion if defined as "non-flat frequency response"