In the 80s alternative meant non-mainstream rock. Like if you didn't like the bands getting mainstream radio airplay, these more obscure bands were your alternative. In the 90s alternative seemed to mean mainstream rock music made by acts that either weren't mainstream in the 80s or didn't exist in the 80s. I'm trying to think of what rock in the 90s was not described as alternative. U2? Bryan Adams?
It seems to be a catch all for grunge, bands inspired by grunge, industrial metal, groove metal, funk metal, punk revivalists, and whatever the hell bands like the Spin Doctors were doing.
Personally I haven't used the term since I was 14.