Perhaps the first person to define Shi’ism from the authors of books on sects (besides the Shia authors) was Imam al Ash’ari. He states:
انما قيل لهم الشيعة لانهم شايعوا عليا رضوان الله عليه و يقدمونه على سائر اصحاب رسول الله صلى الله عليه و سلم
They were only referred to as Shia because they supported ‘Ali radiya Llahu ‘anhu and granted him precedence over all the other Sahabah of Rasulullah salla Llahu ‘alayhi wa sallam.
This definition fits perfectly upon the Mufaddilah sect of the Shia (those who grant ‘Ali precedence over Abu Bakr, ‘Umar and the rest of the Sahabah of Rasulullah salla Llahu ‘alayhi wa sallam). However, the Ithna ‘Ashariyyah do not accept that merely believing that he was superior to everyone else is enough to be called a Shia. It is incumbent, according to them, to believe that he was divinely appointed as the khalifah… it began as soon as Rasulullah salla Llahu ‘alayhi wa sallam passed away… it is for this very reason that al Tusi and al Mufid excluded some of the Zaidiyyah from the boundaries of Shi’ism (as was explained). It could be correct to say that the definition of al Ash’ari includes all or most of the sects of the Shia, without confining it to those who believe in divine appointment as assumed by the Rawafid.
Among the most precise definitions of the word Shia (according to some) is the definition of Ibn Hazm. He states:
و من وافق الشيعة فى ان عليا رضى الله عنه افضل الناس بعد رسول الله صلى الله عليه و سلم و احقهم بالامامة و ولده من بعده فهو شيعى و ان خالفهم فيما عدا ذلك مما اختلف فيه المسلمون فان خالفهم فيما ذكرنا فليس شيعيا
Whoever agrees with the Shia in accepting that ‘Ali was the most virtuous person after Rasulullah salla Llahu ‘alayhi wa sallam and he was most deserving of the position of Imamah and the same applies to his progeny after him, then he is a Shia, even if he disagrees with them in other matters regarding which the Muslims have had difference of opinion. If he opposes them regarding that which we have mentioned, then he will not be a Shia.
This definition was accepted by one of the Rawafid as the preferred definition, discarding that which his cohorts have stated. He considered it the most precise definition of the word Shia. He explains the reason behind this choice of his saying, “from the reasons which impelled us to grant preference to the definition of Ibn Hazm is that accepting the superiority of Imam ‘Ali over the rest of the people after Rasulullah salla Llahu ‘alayhi wa sallam, accepting that he was the Imam and Khalifah after Rasulullah salla Llahu ‘alayhi wa sallam and believing that the same applies to his progeny is the foundation and core of Shi’ism.”
However, if anyone reads the statements of the Shia as far as their beliefs are concerned, such as Imamah, ‘ismah (infallibility), Taqiyyah, etc., he will find that they exaggerate the importance of each of their beliefs, to the extent that they declare adherence to these beliefs a pre-requisite for being included among the Shia. This has already been explained above. Perhaps al Shahrastani realised this when he defined for us the word Shia. His definition is the most comprehensive and inclusive definition with regards to the principles of the Shia.
Al Shahrastani says:
الشيعة هم الذين شايعوا عليا رضى الله عنه على الخصوص و قالوا بامامته و خلافته نصا و وصية اما جليا و اما خفيا و اعتقدوا ان الامامة لا تخرج من اولاده و ان خرجت فبظلم يكون من غيره او بتقية من عنده وقالوا ليست الامامة قضية مصلحية تناط باختيار العامة و ينتصب الامام بنصبهم بل هى قضية اصولية و هى ركن الدين لا يجوز للرسل اغفاله و اهماله و لا تفويضه الى العامة و ارساله
ويجمعهم القول بوجوب التعيين و التنصىص و ثبوت عصمة الانبياء و الائمة و جوبا عن الكبائر و الصغائر والقول بالتولى و التبرى قولا و فعلا و عقدا الا فى حال التقية و يخالفهم بعض الزيدية فى ذالك.
The Shia are those who support ‘Ali radiya Llahu ‘anhu exclusively. They believe that he was divinely appointed through the bequest of Nabi salla Llahu ‘alayhi wa sallam (either directly or indirectly) as the Imam and Khalifah. They also believe that Imamah is confined to his progeny. If anyone else holds the position, he does so through oppression or on account of the Imam adopting Taqiyyah. They claim that Imamah is not just a governmental position which can be attained by selection or appointment by the public. Instead, Imamah is a pivotal matter and it is the basis of religion. The messenger is not allowed to be negligent regarding it, overlook it, hand it over to the masses or leave it suspended.
The common factor between them is the belief in the theory that divine appointment took place, the prophets and the Imams were divinely protected from all major and minor sins and the belief that it is incumbent to practice Tawalli (support for ‘Ali radiya Llahu ‘anhu and those who they assume are his supporters) and Tabarri (disassociate from all those who they falsely accuse of being the haters of ‘Ali radiya Llahu ‘anhu) in word, practice and in dealings, except under the pretence of Taqiyyah. Some of the Zaidiyyah disagree with them.
This definition has made it clear that all the sects of the Shia, besides some of the Zaidiyyah, believe in the incumbency of believing in Imamah, ‘ismah, Taqiyyah. The Imamiyyah, as will be seen, believe in a few more concepts such as Ghaybah, Raj’ah, Bada, etc. Similarly, it is important to note that Imam Zaid and his followers do not believe in the infallibility of the Imams. In addition, they do not prevent the ummah from appointing an Imam for themselves. This is why Imam Zaid considers it permissible for a less virtuous person to be the Imam in the presence of those who surpass him in virtue. They do not believe in Taqiyyah. It seems as if al Shahrastani was indicating towards this when he said, “some of the Zaidiyyah disagree with them.” However, there are some of the Zaidiyyah who believe that Fatimah, ‘Ali and Hussain radiya Llahu ‘anhum were infallible. Others believe that three Imams (‘Ali, and his two sons radiya Llahu ‘anhum) were divinely appointed. These beliefs are against the beliefs of the majority.
 Abu al Hassan ‘Ali ibn Ismail ibn Abi Shibr. He was from the progeny of the great Sahabi of Rasulullah salla Llahu ‘alayhi wa sallam, Abu Musa al Ash’ari radiya Llahu ‘anhu. He was born in the year 260/270 A.H and he passed away in the year 324 A.H in Baghdad. He was unparalleled in the science of polemics and beliefs, to the extent that the Ahlus Sunnah have accepted him as an imam in this field. “Abu al Hassan was extremely intelligent, he was an ocean of knowledge and he accomplished many great tasks. He authored many books which reflect the vastness of his knowledge.” For more details refer to al Dhahabi: Siyar A’lam al Nubala’ 15/85-90.
 Maqalat al Islamiyyin 1/65
 Abu Muhammad ‘Ali ibn Sa’id ibn Hazm al Zahiri. He was born in the year 383/4 A.H in Cordoba and he passed away in the year 456 A.H in Andalus (Spain).
 Al Fisal 2/107
 ‘Abdullah Fayyad: Tarikh al Imamiyyah pg. 33
 Muhammad ibn ‘Abdul Karim ibn Ahmed Abu al Fath, commonly referred to as al Shahrastani. Imam al Subki says: “He was an outstanding Imam in the fields of Islamic philosophy and logic. He had expertise in jurisprudence, principles and Islamic philosophy. Among his writings are Al Milal wa l-Nihal, Nihayat al Iqdam, etc.” He was born in the year 467 or 479 A.H. and he passed away in the year 548 A.H. Refer to Tabaqat al Shafi’iyyah 6/128-130, Mir’at al Jinan 3/284-290
 Al Milal wa l-Nihal 6/146
 Ibn al Murtada: Al Bahr al Zakhkhar pg. 96, Al Muqbili: Al Mu’allim al Shanih pg. 386, Ibn ‘Ibad: Nusrat Mazahib al Zaidiyyah pg. 164-169
 Yahya ibn Hamzah: Al Risalah al Wazi’ah pg. 28
 Al Samarqandi: Al Mu’taqadat (scroll 35 of the manuscript).