A precise definition of Hizbiyyah-By Shaikh Rabee (may Allaah preserve him)
The Shaikh (may Allaah preserve him) stated:
Having a bigoted partisanship for a particular idea that opposes the Book of Allaah and the Sunnah of the Messenger, and (establishing) allegiance and enmity upon it, this is Tahazzub (illegal partisanship). This is Tahazzub (illegal partisanship) even if is not in (the form of an) organisation. To formulate deviated ideas and gather the people on it, this is a hizb, regardless whether it is (in the form of an) organisation or not. As long as it opposes the Book and Sunnah (then it is a hizb).
[Source: Kash As-Sitaar page 19-20. Quoted from question number 8]