This word lives a double life. On the one hand, it can describe something that is legitimately popular because it is good. However, through the hipster douche
movement, the phrase "this ___ is mainstream" is used almost as often as "I love PBR" and "I need some more baby powder and Crisco to fit in my jeans."
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.
One definition: "The band broke into the rock mainstream with its smash hit (insert song here)."
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."