Definition of Kharijites:
The Kharijite in the language is the plural form of Kharij (rebel) and the word was mentioned in the language books to indicate the groups of opinions and desires because they rebel against the religion or against Imam Ali (may Allah be pleased with him).
Al Azhary said: The Kharijite is a group of people who follow their desires. (1)
Az-Zabidy said about them: They are the Haruriyah, and the Kharijite are a group of them. They were seven groups and were named as such because they rebelled against the people, against the religion, against the truth, or against Ali (may Allah be pleased with him) after the Battle of Siffin. (2)
Idiomatically: Ash-Shahristany said: Everyone rebels against the true Imam about whom the Muslim group agreed is called a Kharijite whether the rebellion was in the time of the Companions against the Rightly-Guided Imams or after them against those who followed them righteously and all imams any time. (3)
The origin of the Kharijites was a group of political trend and special thinking who rebelled against Imam Ali (may Allah be pleased with him) after he was pleased with the Judgment that took place after the Battle of Siffin and fought them in the famous Battle of Nahrawan. (4)
Despite the scatter of the Kharijite into different sects, they hold the same doctrine which stated:
1- They free themselves of `Uthman and Ali (may Allah be pleased with them), and advance that on every act of obedience. Moreover, they held the marriage contracts valid on that fundamental otherwise they are invalid.
2- They call the people of grievous sins disbelievers.
3- They held that rebellion against the Imam is a must when he contradicts the Sunnah.
However, `Abdul-Qadir Al Baghdady had a different opinion in the things which combined the Kharijite. He said: "The correct view in the things that combine the Kharijite is the narration of Sheikh Abu Al Hasan Al Ash`ary (may Allah bestow mercy on his soul):
They called Ali, `Uthman, the people who fought in the Battle of Al Jamal, the two judges, anyone who approved their ruling, or approved one of them, or even satisfied the judgment disbelievers." (5)
Abu Al Hasan Al Ash`ary said in Maqalat Al Islamiyyin: The Kharijites hold consensus on calling `Ali ibn Abu Talib (may Allah be pleased with him) a disbeliever. However, they disagreed whether his disbelief was part of polytheism or not? They also hold consensus that every major sin is an act of disbelief except the sect which was called An-Najdat who did not held that view. They also hold consensus that Allah (Glory be to Him) tortures the people of grievous sins permanently except An-Najdat.
(1) Tahdhib Al Lughah (7 / 50).
(2) Taj Al `Arus (2 / 30).
(3) Al Milal Wa An-Nihal (1 / 114).
(4) See: Al Fisal Fi Al Milal Wa An-Nihal (4 / 157), Al Milal Wa An-Nihal (1 / 21), Al Bidayah Wa An-Nihayah (7 / 189), Firaq Mu`asirah (1 / 70, 71).
(5) Al Farq Bayna Al Firaq p. 74.