Confession time:
It's been like 35 years and I still don't have a single fucking clue what the term "Alternative Music" means.
I think it used to me "you will only hear this on American college radio" but then bands like REM, The Pixies, and Nirvana started cranking out chat topping mainstream hits and I don't think the word means anything right now. "Alternative Rock" is literally the most meaningless term in music categorizing.