Kharijites were the brain dead folks at Siffin

Definition
"Whoever rebelled against the legitimate imam accepted by the people, whether this rebellion took place at the time of the companions against the rightly guided Imams or against their worthy successors or against the Imams at any time" (Al-Shahrastani, al-milal wa al-nihal page 98).

History
At Siffin the Kharijites compelled Ali to accept the arbitration. Then when Ali wanted to appoint abdullah b. Abbas as his arbitrator, the Kharijite objected, saying "He is a relative of yours".

They (Kharijite) forced him (Ali) to send instead Abu Musa Al-Ashari to judge according to the book of God. The arbitration went against Ali and when Ali did not accept it, they rebelled against him.

"Why did you appoint men as judges?", they asked. "Judgments belong to God alone". These were the rebels who afterward assembled at Nahrawan. (Al-Shahrastani, al-milal wa al-nihal page 99).