“Indeed Abraham was among his followers [shī‘ah]”
and the verse,

“The one who was from his (Moses’) followers [shī‘ah] sought his help against him who was from his enemies.”
The word “Shī‘ah” has also been mentioned in the Prophetic traditions to mean “followers and friends of ‘Alī (‘a)”.
“Shī‘ah” in the Shī‘ah references does not have more than one meaning and conception and that is belief in the succession of ‘Alī (‘a) and his eleven descendants in which no change has ever taken place since the demise of the Holy Prophet (S)up to the minor occultation [ghaybah aṣ-ṣughrā]. Just as the Shī‘ah of the second half of the third century hijrī believed in all the twelve Imāms (‘a), the pioneering Shī‘ah among the Companions of the Holy Prophet (S)also believed in this affair because they had been informed of the names of these Imāms from the Prophetic traditions.
Although many of the Shī‘ah had no access to these traditions due to the atmosphere of strangulation maintained by the tyrant rulers, what was obligatory (for them) was to recognize the Imām of their respective times. As the Holy Holy Prophet (S)said, “He who will die without recognizing the Imām of his time dies in a state of ignorance [jāhiliyyah].” As such, we can see that when Imām aṣ-Ṣādiq (‘a) attained martyrdom, Zurārah who was an old man sent his son, ‘Ubayd, to inquire about the successor of Imām aṣ-Ṣādiq (‘a). But before ‘Ubayd was able to return to Kūfah, Zurārah, who was about to die, took hold of a copy of the Qur’an and said: “O God! Be my witness that I testify to the Imamate [imāmah] of the one who has been designated in this Qur’an.”
Of course, with the passage of time, the meaning and concept of Shī‘ah assumes an explicit form and its scope is determined. Thus, the infallible Imāms (‘a) have regarded those who are identified with the false sects and faiths as outside Shī‘ah circles, as Shaykh aṭ-Ṭūsī narrates from Ḥamrān ibn A‘īn:
I asked Imām al-Bāqir (‘a): “Am I really among your Shī‘ah?” The Imām (‘a) replied: “Yes, you are among our Shī‘ah in this world and in the hereafter, and the names of the Shī‘ah and their fathers are written for us. Why, are there those who turn their back to us?” I replied: “May I be your ransom! Is it possible for somebody to be your Shī‘ah and to have knowledge of your being in truth, and then to turn his back from you?” The Imām (‘a) said: “Yes, O Ḥamrān! You will not perceive them.”
Ḥamzah az-Zayyāt, who is one of the narrators of this hadith, thus says:
Concerning this hadith we made a discussion and we were not able to understand the purport of the Imām (‘a). As such, I wrote a letter to Imām ar-Ridā (‘a) and I asked him (‘a) (about this). The Imām (‘a) said: “Imām aṣ-Ṣādiq (‘a) was referring to the Wāqifiyyah (a deviant sect).”
It is for this reason that in the parlance of the Shī‘ah rijāl writers, the title Shī‘ah is only applied to the Shī‘ah believing in the twelve Imāms (‘a) and in the language of the jurists [fuqahā], they are sometimes described as “our companions” [aṣḥābunā] or “our Imāmī companions” [aṣḥābunā al-imāmiyyah]. And those who had inclined toward the deviant sects and drifted away from the course of Shī‘ism have described with such labels as Faṭḥī, Wāqifī, Nāwūsī, etc. and if ever the names of some of them are mentioned in the Shī‘ah books on rijāl, the reason is that they had narrated these traditions prior to their deviation, just as the names of a number of Sunnī narrators who have narrated from the infallible Imāms (‘a) have been mentioned in these books.
The Sunnī scholars and rijāl writers, however, have used the word Shī‘ah in broader sense and they have applied it to all the sects that have separated from the body of Shī‘ism and even to the ghulāt as well.
In addition, they also refer to those who love and admire the Ahl al-Bayt (‘a) as “Shī‘ah”. This is while some of these people do not have any sort of belief in the infallibility [‘iṣmah] and Imamate [imāmah] of the Ahl al-Bayt (‘a), such as Sufyān ath-Thawrī, a rector [muftī] in Iraq who issued edicts [fatāwā] based on the Ahl as-Sunnah, but Ibn al-Qutaybah has enlisted him along with the Shī‘ah. Regarding ash-Shāfi‘ī, who is the founder of one of the four Sunnī schools of thought [madhāhib], Ibn Nadīm thus says:

.
“Ash-Shāfi‘ī had extreme Shī‘ism [tashayyu‘].”
Of course, during the second and third centuries hijrī, besides the Shī‘ah Imāmī, the Zaydīs constituted the greatest number of Shī‘ah. They were “Shī‘ah” more in the political sense than in ideology because, in terms of jurisprudence [fiqh], they were not followers of the Ja‘farī fiqh; rather they were followers of the Ḥanafī fiqh. From the viewpoint of the ideological principles also, as narrated by Shahristānī, “For sometime, Zayd was a student of Wāṣil ibn ‘Aṭā’, the founder of the Mu‘tazilah (Mu‘tazilite) madhhab and has learned from him the principles of the Mu‘tazilah madhhab.”
Therefore, the Zaydīs are Mu‘tazilites in principles [uṣūl]. It is for this reason that they used to regard as permissible [jāyiz] the Imamate [imāmah] of a deserving person [mafḍūl] in the existence of the more deserving person [afḍal] and in that they do not disrespect the two sheikhs [shaykhayn] (Abūbakr and ‘Umar). And in terms of beliefs, they are closer to the Ahl as-Sunnah, as Ibn al-Qutaybah thus says: “Among the rāfiḍī (Shī‘ah) sects, the Zaydīs have the least extremism [ghulū].”
It was for this reason that the uprising of Muḥammad Nafs az-Zakiyyah—one of the Zaydī leaders—was praised by some jurists [fuqahā] of the Ahl as-Sunnah, and as narrated by Wāqidī, Abūbakr ibn Sīrah, Ibn ‘Ajlān, and ‘Abd Allāh ibn Ja‘far —who were among the great ḥadīth scholars [muḥaddithūn] of the Medina school [maktab] and from whom Wāqidī himself has narrated ḥadīth—were involved in the uprising of Muḥammad Nafs az-Zakiyyah. Also, Shahristānī says: “Abū Ḥanīfah was among the followers of Muḥammad Nafs az-Zakiyyah.”
The Mu‘tazilites of Baṣrah also agreed with the uprising of Muḥammad and based on Abū’l-Faraj al-Iṣfahānī’s narration, “A group of the Mu‘tazilites in Baṣrah such as Wāṣil ibn ‘Aṭā’ and ‘Amrū ibn ‘Ubayd have paid allegiance to him.”
As such, the Zaydīs can be regarded as Shī‘ah only from the political viewpoint although they believe in the superiority of the descendants of Fāṭimah (‘a).

 Lesson 3: Summary
Shī‘ah, according to the lexicon, refers to the followers and supporters of ‘Alī (‘a). In the Shī‘ah references, “Shī‘ah” does not have more than one meaning and that is belief in the succession of ‘Alī (‘a) and his eleven descendants. The infallible Imāms used to consider those who were identified with the deviant sects as outside the circle of Shī‘ism, but the Sunnī scholars and rijāl writers have used the word Shī‘ah in broader sense and they have applied it to all the sects that have separated from the body of Shī‘ism as well as to the admirers of the descendants of the Prophet