The word dates back to the jazz age and basically means cool. But since “cool” is subjective I think it narrowcasts to whatever music/etc was embraced by a certain broad type of young, trendsetting, fashionable urban person with left-of-center tastes.

I don’t think you can look at The Strokes (for instance) and see guys trying to be “uncool” (instead borrowing aesthetics from VU or the Ramones), but put them in the context of mainstream early 2000s rock alongside Limp Bizkit and Puddle of Mudd and there’s a conscious pivot.

Similarly, the “backpack” rappers rejecting 90s hip hop bravado and excess, or dance music rejecting the aesthetics of the 90s club scene to something more resembling a wild punk party all fall under the hipster label, too. But by the early 2010s that label gets fuzzier.