Someone who changes their musical style/goes to a bigger record label to make more money. People just say sellout so they can look hardcore though. Because let's look at blink-182, they changed their style, but they've been on the same label. Did they sellout ? Who knows ? Maybe they just wanted to change their style from the old less-mature stuff.
People say Greenday sold out with their CD, "Dookie" from changing labels, but they didn't change their style.