According to many hipsters (or at least the obnoxiously arrogant ones), any musical artist that is signed to a label, sells music online or attracts more than a dozen people to his/her show is "mainstream," a "sellout," or any other word they found in their thesaurus they bought with their parents money.
The newer definition: "Screw (insert band name here), that band is so mainstream. Come on, guys. Let's all dress in an identical manner and listen to music Pitchfork tells us is cool."
The only inkling of individuality is either a) the lead singer's voice, or b)the fucking tone of the guitar.
Both of those are still a rarity.
Sometimes, you can define a mainsrteam band by the ranking they are at on the top 40 list, or the iTunes most popular. But not always, because sometimes good indie music is mistakenly put there.
smart indie kid: "Fuck man, don't spend money on that mainstream garbage.
Go buy some Pinback."