The act of forcing someone to listen to your favorite bands, or bands you "think they would like" in an effort to justify your musical tastes. This usually happens when first meeting someone whose musical tastes are unknown to you.
Joey: How was the car ride?
John: It was okay, except Alli insisted on playing her iPod at full blast the entire time "showing" me all these bands I would supposedly like. I like music and all, but it eventually got obnoxious.
Joey: Well golly gee willikers, that sounds like a textbook example of Musical Imperialism.
Everyone knows that guy/girl who always comes to your party and stealthily plugs in their own iPod between songs. You were bumping old school Outkast, but now you have to settle for "She Wolf" by Shakira. You shrug it off because you're winning at pong at the moment, but let's be honest, that's musical imperialism.
Josey: Hey, man, I really like this country playlist. It's totally appropriate for the setting and provides a really good vibe....wait, who the fuck just put on Ludacris?
Hal: It's Ashley. She's always stealing the stereo.
Josey: Dude! That's musical imperialism at its worst right there.