Alternative Music might best be understood contextually and historically. Back in the mid-to-60's (and the early 70's), commercial stations mostly played whatever was popular....at the cash register. Where could you go for an alternative to the innocuous, or maybe noxious stuff aired. Mostly low-wattage college FM stations, though there were a few for- profit broadcasters in the more audio-artistically sophisticated cosmopolitan locales.
San Francisco, Washington, DC, and New York provided outlets for hearing the newest, the best, and the brightest that underground music, progressive rock or jazz, folk, blues, new R and B, and electronic musicians were producing and recording.
WAMU (of American University) had its "Rock and Roll Jukebox show" that played esoteric hard rocking bands that no other local station had ever heard of (or would play, for that matter.} They, still to this day, play Bluegrass and "roots of Country Music", (and Gospel "Stained glass Bluegrass" on Sunday mornings).
Georgetown University's WGTB was playing a variety that could not be touched on any other broadcast-- especially playing the latest from California and NYC, as well as local avant garde groups.
Commercially, WHFS (originally in Bethesda, MD, now Annapolis, MD), in that early era, provided a welcome hip option on the dial.