The process of converting copyright-protected .m4p files purchased at the iTunes music store into more flexible file formats such as .mp3 so they can be used devices other than iPods and authorized computers running iTunes.
This can be done using 3rd-party iTunes hacking software, or by burning the songs to an audio CD and ripping them back to the computer.
"Shit, now that I've downloaded $100 of music from the iTunes store, I'm going to have to go through De-iTunification to play these files on my sony mp3 player."
"I'd send you that song, but you wouldn't be able to play it. It hasn't gone through De-iTunification yet."