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1.
The act of shitty music. Rubbish music with no real meaning in the words - normall aimed at 9-12 year old girls who read Smash Hits and stuff. See Blazin Squad and Westlife

Irritating girl: WOW WESTLIFE CAME ON THE TV!!!! I LOVE YOU SHANE
Normal person: turn that musical terrorism off
by Who cares? October 13, 2005
 
2.
The act of instilling repetitive, catchy music/jingles into someone's head. Usually songs that you know will get stuck into someone's head like "Friday" by Rebecca Black or the Oscar Meyer jingle.
Someone instilling musical terrorism could do so by repeating the same verse of a song like Ke$ha's "throw some glitter make it rain" everytime glitter or rain is mentioned.
by GapStockCrew June 03, 2011