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1.
The unauthorized replacing of the original soundtrack of an audiovisual recording, such as a movie or television show, with one designed to substantially alter the original plot and/or characters.

The term "trackjacking" is derived from a conjunction of the phrase “soundtrack hijacking,” and is intended to convey the idea of unauthorized usurpation.

A trackjack is easily distinguishable from a mashup. A mashup combines two preexisting works in a creative way to make a new product through selection and arrangment. A trackjacker adds their own new content to a preexisting work while leaving the preexisting work substantially unaltered.
My friends and I got together last weekend to do some trackjacking. We recorded new dialogue to "My Little Pony: Princess Promenade." You should rent the movie, then watch it on mute while listening to our trackjack on your iPod.

Examples of trackjacking include:

Brad Neely's "Wizard People, Dear Reader"
My Way Entertainment's "The Juggernaut Bitch!!"
Fensler Films' "G.I. Joe PSAs"
by Patriarch917 February 27, 2007