As I wrote that is only one definition.

Just an example:
"[...]I use '8-bit sound' term to define chiptune/chipmusic because everybody do it.[...]"
Who is "everybody"?! I never heard/read about this. If someone is saying that he/she is simply wrong.

"8-bit sound" is 8-bit sound as "16-bit sound" is 16-bit sound and so on. I has nothing to do with music style.
Chiptune/chipmusic have (not) to use a sound chip.
Sample based music that sounds like chiptune/chipmusic is (not) chiptune/chipmusic.