Can the term shadow pertain to anything else than light? Feel free to interpret this question in the widest sense possible.
Yes! Any beam that is blocked by an object will basically make a shadow. For example, the IceCube detector can see the moon's cosmic ray shadow.
For example, acoustic shadow (http://en.wikipedia.org/wiki/Acoustic_shadow ).
Yes, the term "shadow" can refer also to something or (dare I say) someone that is dark, shady, inconspicuous, etc. One can also use it as a verb; to shadow someone is to follow them closely. Like "I'm having the new guy shadow me for a while until he learns how to do everything".