It is the abbrevation for Partiya Karkeran Kurdistan (Kurdistan Workers Party), which fights for an independant Kurdish state. PKKs leader, Abdullah Ocalan, is detained in an island prison, in Turkey, since he was captured, 15 Feb. 1999, with the help of CIA and MOSSAD, by turks.
Did you know that, by posting this definition on urbandictionary.com, I am breaking Britain's absurd anti-terrorism laws by creating a document which bears the name of a terrorist group? Also that you the reader, and the owners of the website, are also breaking it?