The term “Ahlul-Bayt” is a Quranic, hadithic, and theological term that refers to the household of the prophet (pbuh). It has been used in the Quran for this meaning once, in surah Ahzab, verse 33: “انما یرید الله لیذهب عنکم الرجس اهل البیت و یطهرکم تطهیراً” (Indeed Allah desires to repel all impurity from you, O People of the Household, and purify you with a thorough purification).
The root word “اهل” means nearness and kinship and the word “بیت” means where one lives and spends the night at. The term “اهل البیت” literally refers to those whom one is family with; it has also been used for a broader meaning that covers all people that one shares religion, abode, city or has family ties with.
Nevertheless, theologians, commentators of the Quran and narrators of traditions believe this term to have a more specific meaning when it is used in the Quran. According to their viewpoints, there are many hadiths from the prophet (pbuh) and imams (as) that clarify what is meant by the term in the Quran.
Of course, there is a difference of opinion amongst commentators, theologians and narrators of hadiths as to whom this specific interpretation of the term is speaking of:
1- Some Sunni scholars say that considering the phrases before and after this term which are speaking of the wives of the prophet (pbuh), the verse containing the term “Ahlul-Bayt” is only referring to the wives of the prophet (pbuh) as well. In order to back this theory, they refer to a hadith by Ibn Abbas narrated by Akramah, Muqatil, Ibn Jubair and Ibn Sa’eb and write that Akramah would shout in the bazaars saying: “The Ahlul-Bayt of the prophet (pbuh) are only his wives and I am prepared to do Mubahilah (malediction) with anyone who denies it!”
Some Sunni commentators of the Quran and all Shia commentators have challenged this theory and said that if the verse was speaking of the prophet’s (pbuh) wives, it would have been appropriate to use the same pronoun to address them (the feminine plural form) that was used before and after this phrase of the verse. The result would be the verse being like this: “انما یرید الله لیذهب عنکن الرجس اهل البیت و یطهرکن تطهیراً”. Yet the truth of the matter is that the verse isn't like this and the pronouns used to address suddenly change, signifying a change in the addressee, hence the falseness of the abovementioned theory.
Add to all of the above the fact that the hadith they have referred has also been questioned; Abu Hayyan Gharnati, a Sunni scholar, writes that this hadith isn't from Ibn Abbas. Ibn Kathir also says that if what is meant by this hadith is that the wives of the prophet (pbuh) were the purpose of the revelation of the verse (Shanul-Nuzul), it is correct, but if what is meant is that the verse is speaking of the wives of the prophet (pbuh) [and that the term “Ahlul-Bayt” is referring to them] it is incorrect because there are many hadiths that reject this assertion.”
Of course, taking into consideration what was said before, the part of Ibn Kathir’s viewpoint that says that the Sha’nul-Nuzul of the verse are the wives of the prophet (pbuh) is also incorrect, because first of all, such a thing isn't in harmony with the order and flow of the verse (Siyaq), and secondly because it contradicts many hadiths that say otherwise that he himself has accepted and admitted to their authenticity.
2- Another group of Sunni commentators believe the verse to be speaking of the wives of the prophet (pbuh), Ali (as), Lady Fatimah (as), Hasan (as) and Husein (as). It should be noted that this group hasn’t referred to any hadith to substantiate this opinion.
3- Some commentators say: “The verse seems to be a general one and covers all of the prophet’s (pbuh) household; his wives, children, close ones and even his male and female servants. Tha’labi says: “It also covers all of the Bani Hashem tribe or the believers from this tribe.” This viewpoint also isn't backed by any hadiths.
4- One group of commentators has said that what might be meant by “Ahlul-Bayt” in the verse are all of those whom giving charity to is forbidden. This view stems from a hadith by Zayd ibn Arqam. He was asked about who the “Ahlul-Bayt” are and if the prophet’s (pbuh) wives are part of them or not; he answered: “The wives of the prophet (pbuh) are part of his household, but what the term is referring to is all of those whom giving charity to is forbidden; meaning the progeny of Ali (as), Aqil, Jafar and Abbas.” According to Abul-Futuh Razi, this is a very rare and unheard opinion that has no reasoning to back it.
5- All Shia and most Sunni commentators strictly believe the “Ahlul-Bayt” that the verse mentions to be the “Companions of the Cloak” (Ashabul-Kisa’) which are Prophet Muhammad (pbuh), Ali (as), Lady Fatimah (as), Hasan and Husein (as) and that the verse has been revealed for them. This viewpoint is backed by clues and proof and numerous hadiths by the prophet (pbuh), Imam Ali (as), Imam Hasan (as), Imam Husein (as), Imam Sajjad (as), other imams (as), Umm Salmah, Aishah, Sa’eed Al-Khadari, Ibn Abbas and other companions of the prophet (pbuh).
The only question that might come to the mind of some is why this phrase is located between verses that are stating the responsibilities of the prophet’s (pbuh) wives and don’t seem to have anything to do with this portion of the verse?
There are different responses to this question. Tabarsi says: “This isn't the only case in which there are verses on different subjects next to each other in the Quran; the Quran is full of cases like this one. Also, there are many examples of such in the words and poetry of great literate Arabs.”
Allamah Tabatabai adds something else to this answer and writes: “There is no reason to say that the phrase “انما یرید الله لیذهب عنکم الرجس اهل البیت و یطهرکم تطهیراً” and the other verses were all revealed together. As a matter of fact, what hadiths clearly tell us is that this portion of these verses was revealed separately, and was placed next to them by the order of the prophet (pbuh) or later on after his passing when the Quran was being compiled.”
In Tafsir Nemouneh, it has been stated that: “A third answer can also be given to this question which is the Quran wants to tell the wives of the prophet (pbuh) that they are living amongst a family in which some of them are infallible; one who lives amongst such people should be more careful of herself and should never forget that being part of a family that contains five infallibles entails great responsibilities, and that Allah (swt) and His creations all have expectations of them [as a result].”
The hadiths about the reason for the revelation of this verse and its true meaning are very high in number and can be categorized into the following:
a) Those that directly and clearly say what the reason for its revelation and who the “Ahlul-Bayt” are.
b) Hadiths that confirm the famous hadith of Kisa’ (the cloak). There are hadiths by Abu Sa’eed Al-Khadari, Anas ibn Malek, Ibn Abbas, Abul-Hamra’, and Abu Burzah that after the event of the Kisa’ and the revelation of the verse of Tathir (the verse being discussed here), the prophet (pbuh) would go to Imam Ali (as) and Lady Fatimah’s (as) home, either at the morning prayer or all five prayer times, for one month, forty days, six or nine months (the numbers are disputed) and say: “السلام علیکم اهل البیت و رحمة الله و برکاته الصلاة یرحمکم الله” and would then recite the verse of Tathir.
Therefore, it is for sure that what is meant by “Ahlul-Bayt” in verse 33 of surah Ahzab are the prophet (pbuh), Ali (as), Lady Fatimah (as), Hasan (as) and Husein (as) according to hadiths. As for why this term has been used for the imams after Imam Husein (as) as well in Shia traditions, one must say:
The reason why the Quran emphasizes on the unique and significant status of the imams is for the people to refer to them for guidance and follow them for perfection, because the main objective the Quran pursues is the perfection of man (الم*ذالک الکتاب لا ریب فیه هدیً للمتقین). If one concentrates a bit on Quranic verses, he will be able to accept this fact, and since the imams were the leaders and guides of the Muslim nation, the title of “Ahlul-Bayt” has been given to them as well. It is because of this that when the prophet (pbuh), who in reality is the true interpreter of the Quran and the one to explain it, wants to speak of the imams being the ones to be referred to after him for guidance and leadership, he says:
“انی تارک فیکم الثقلین کتاب الله و عترتی ما ان تمسکتم بهما لن تضلوا بعدی ابداً فانهما لن یفترقا حتی یردا علی الحوض” (I am leaving two weighty and important things with you [after me]; the book of Allah (swt) and my household and Ahlul-Bayt, if you cling to them you will never go astray; these two will never part until they come to me next to the pool of Kawthar on the Day of Judgment), or says:
“انما مثل اهل بیتی فیکم کسفینة نوح من رکبها نجا و من تخلف عنها غرق” (Indeed my Ahlul-Bayt are like the ship of Nuh, whosoever boards it will be saved, and whosoever stays behind and doesn’t, drowns).
For further information, see:
1- The Ahlul-Bayt and verse 23 of surah Shura, Question 160 (website: 1229).
2- The Ahlul-Bayt of the prophet of Islam (pbuh), Question 833 (website: 902).
3- The instances of the Ahlul-Bayt, Question 1247.
4- Allocating the Ahlul-Bayt to the “Companions of the Cloak”, Question 159, (website: 1251).
 Fuyumi, Misbahul-Munir, pg. 28.
 Ibid, pg. 68.
 Isfihani, Mufradat Ragheb, pg. 29.
 Tabari, Jame’ul-Bayan, vol. 22, pg. 7; Ibn Kathir, Tafsirul-Quranil-Adhim, vol. 6, pg. 365.
 Tafsir Ibn Kathir, vol. 5, pp. 452-453.
 Fakhruddin, Tafsir Kabir, vol. 25, pg. 209; Beydhawi, Anwarul-Tanzil, vol. 4, pg. 163; Abu Hayyan, Al-Bahrul-Muhit, vol. 7, pg. 232.
 Qurtubi, Al-Jame’ Li-Ihkamil-Quran, vol. 14, pg. 183; Alusi, Ruhul-Ma’ani, vol. 22, pg. 14.
 Abu Hayyan, Al-Bahrul-Muhit, vol. 7, pp. 231- 232.
 Abul-Futuh Al-Razi, Ruwadhul-Jinan wa Rowhul-Jinan, vol. 15, pg.
 Majma’ul-Bayan, vol. 7, pg. 560.
 Al-Mizan vol. 16, pg. 312.
 Tafsir Nemouneh, vol. 17, pg. 295.
 Tabari, vol. 22, pp. 6-7; Qurtubi, vol. 14, pg. 183; Hakem, vol. 2, pp. 416 and 3146; Bukhari, Al-Tarikh, vol. 1, (2), pp. 69-70; Tirmidi, Al-Sunan, vol. 5, pg. 663.
 Tabari, vol. 22, pp. 6-7; Bukhari, Al-Kuna, pp. 25-26; Ahmad ibn Hanbal, Mustanad, vol. 3, pg. 259; Haskani, Shawahidul-Tanzil, vol. 2, pp. 11-15; Suyuti, Al-Durrul-Manthur, vol. 6, pp. 606-607.
 Mar’ashi, vol. 2, pp. 502-547, vol. 9, pp. 91-92.
 In different parts of this article, Nameye Ahle Bayt, written by Ali Rafi’i Alamarvdashti, has been used, pp. 301-308. For further information, please refer to the book.
 Baqarah: 1 and 2.
 Sahih Tirmidi, vol. 2, pg. 380; Ahmad ibn Hanbal, Mustanad, vol. 3, pg. 17.
 Hakem, Mustadrakul-Sahihein, vol. 2, pg. 432; Firouzabadi, Fadha’ilul-Khamsah, vol. 2, pg. 65.