etymology of the word Khawarij
If the word "khuruj" is used with an indirect object introduced by `ala, it has two meanings which are near to one another. One is to stand up in a position for battle or war, and the other is disobedience, insubordination and revolt. The Arabic dictionary al-Munjid says that "kharaja" with an indirect object introduced by `ala means to come forward to fight someone, or it can be used for subjects rebelling against the king: insurrection.
The word "~khawarij~ ", meaning revolt, comes from "khuruj " in the second sense. That group which evidence the command of ~Ali~ and rebelled against him is called the Khawarij. Since they based their disobedience on a belief and on a religious ideology, they became a sect, and the name came to be used especially for them; and so it was not used for any other people who rose up after them and rebelled against the ruler of their times. If they had not had a particular creed and belief, they would have been like other rebels of the periods after them, but they did have a belief and later on this very belief found some kind of independent existence. Although they never managed to form a government, they did manage to create a school of law and a literature of their own.
There were individuals who never got around to actual rebellion, although they believed in it, as it is said of ~Amr ibn Ubayd~ and other ~Mu'tazilah~. It was said of some of the Mu'tazilah who had beliefs similar to the Khawarij about "bidding to good and forbidding evil", or about the matter of those Muslims who are guilty of moral sin still finding a place in paradise, that "they thought like the Khawarij".
Thus there is a degree of commonality between the lexical meaning of the word and its particular reference.
Sources :
attraction and repulsion of Imam Ali p.b.u.h- pages: 107