What would you call Alternative?

This is a random question, but what qualifies as alternative. It seems like of all the genres out there, alternative really couldn't be more broad. If you tuned into a hard rock station, the songs would be pretty similar, dropped d tuned, similar rhythms, structures, tones, etc. Listen to a pop station, you get the same chord progressions over and over. Turn on hip hop and I dare you to hear a song without autotune. Pick jazz, you get piano and guitar solos with soft drums.

But what is alternative then? I just can't put my finger on it as a genre. I was listening to the Sirius alternative station today and four songs in a row came on and each sounded completely different from the last. It just seems to me that alternative music has pegs farthest from the center of the map.

How would you define alternative music? What are the unifying themes in alt music?