“Hipster” is a term co-opted for use as a meaningless pejorative in order to vaguely call someone else’s authenticity into question and, by extension, claim authenticity for yourself.
It serves no conversational function and imparts no information, save for indicating the opinions and preferences of the speaker.
Meanwhile, a market myth has sprung up around the term, as well as a cultural bogeyman consisting of elusive white 20-somethings who wear certain clothes (but no one will agree on what), listen to certain music (no one can agree on this either), and act a certain way (you’ve probably sensed the pattern on your own). One may be quick to coin such interests and garb as "black-rimmed glasses, flannel, and indie music," however said mainstream-avoiding "hipsters" will logically turn down abiding to such a popular consensus, voiding the interpretation null.
You can’t define what “that kind of behavior or fashion or lifestyle” actually is, nor will you ever be able to. That’s because you don’t use “hipster” to describe an actual group of people, but to describe a fictional stereotype that is an outlet for literally anything that annoys you.
The twist, of course, is that if it weren’t for your own insecurities, nothing that a “hipster” could do or wear would ever affect you emotionally. But you are insecure about your own authenticity - “Do I wear what I wear because I want to? Do I listen to my music because I truly like it? I’m certainly not like those filthy hipsters!” - so you project those feelings.
Suffice it to say, no one self-identifies as a hipster; the term is always applied to an Other, to separate the authentic Us from the inauthentic, “ironic” Them.
tl;dr: if you believe hipsters exist, you are a plebeian.