Dictionary.com quote:

–noun
1. a prevailing custom or style of dress, etiquette, socializing, etc.: the latest fashion in dresses.
2. conventional usage in dress, manners, etc., esp. of polite society, or conformity to it: the dictates of fashion; to be out of fashion.

I would say that fashion is extremely regional. A lot of what is considered chic in New York would probably be considered ridiculous on the West Coast, while we West Coasters would look under-dressed on the East Coast. The image below is actually from New York Fashion Week. While no one, even in New York, would wear that off the runway (I hope!), it's someone's "design aesthetic" that they clearly like, and that they think should be the new trend.

Fashion is also cultural. "Alternative" fashion is the norm amongst that group, so it is their fashion. It speaks to their experience, and to their aesthetic.

I live in a city that is often considered as out-of-fashion as a city can be, because we pretty much accept people wearing whatever they feel like, and not running to get whatever someone tells them they should like. To me, that makes whatever mix we have our fashion. It's our "prevailing custom" to wear what we think works on our body.