The lexical meaning of splitting-up (iftiraaq) is: Fragmentation, which is disunity, separation, cutting-off. It is also derived from the term divergence and abberation. From it comes [the expression]: Departure from the fundamental, or from the path, or from the united body [Consult Lisaanul-'Arab (10/299) of Ibn Mandhoor].
The technical meaning of iftiraaq is: Departing from the Sunnah and the Jamaa’ah in one or more of the decisive (qat'ee) fundamantal precepts (usool) of the Religion - whether it be a precept of belief, or one of the decisive fundamental precepts connected to actions, or to the greater welfare of the ummah.
Abu Hurayrah radiallaahu 'anhu narrated from the Prophet sallallaahu 'alayhu wa sallam who said:
“Whoever leaves obedience and separates from the Jamaa’ah and dies, dies a death of ignorance. Whoever fights under the banner of the blind; becoming angry for partisanship, or calling to partisanship, or assisting partisanship and is killed, is killed [in the cause of] ignorance. Whoever rebels against my ummah, killing both the righteous and the wicked of them, not sparing even the believers, nor those who have been given a pledge of security, is not from me, nor am I from him.” [Reported by Muslim no.1848].
Therefore, opposing Ahlus-Sunnah wal Jamaa’ah in any of the fundamental precepts of belief ('aqeedah) is deemed as splitting (iftiraaq) and separating from the Jamaa’ah. Opposing the consensus (ijmaa) of the Muslim scholars is deemed as splitting and separating from the Jamaa’ah, and opposing the united body of Muslims and their leader, in what is from the [issues of] great welfare, is deemed as splitting and separating from the Jamaa’ah. Departing from the consensus of the Muslim scholars is an act of splitting since it is separating from the Jamaa’ah.
The greater disbelief (kufr akbar) is counted as splitting, whereas not every splitting is disbelief. What is meant by this is that every act or belief which causes a person to leave the fundamentals of Islaam, or any of the decisive matters of the Religion, or from the Sunnah and the Jamaa’ah in that which necessitates disbelief, has indeed separated and split. However, not every splitting is disbelief. Meaning that if a party or a group of people fall into [a matter] of splitting, they are not to be described with disbelief unless they split from the Jamaa’ah of the Muslims in a particular action [which necessitates this]. An example is the splitting of the Khawaarij. For the Khawaanj were the first people to split from the ummah, they rebelled against the ummah with the sword and split from the Jamaa’ah of the Muslims and its leader. However, the Companions did not apply the ruling of disbelief upon them, rather they differed concerning this. That is why when 'Alee radiallaahu 'anhu was asked concerning them, he did not apply the ruling of disbelief to them. The same was the case with Ibn 'Umar and other Companions – may Allaah be pleased with them all - who used to pray behind Najdah, the Khaarijee and Ibn 'Abbaas agreed to debate Naafi' ibn al-Azraq with the Qur'aan; debating with him [on the basis that Naafi' was] a Muslim. [Consult Minaahjus-Sunnah (5/247-248) of Ibn Taymiyyah].