What has reached us through the narrators about the Messenger of Allah (peace be upon him) is called Hadith. Sometimes Hadith is also called Sunnah, Khabar, or Athar.

The hadith in which the statement (saying) of the Prophet (peace be upon him) is mentioned.

The hadith in which the action of the Prophet (peace be upon him) is mentioned.

The hadith in which the silence of the Prophet (peace be upon him) on a matter is mentioned.

The hadiths in which the habits, morals, or physical characteristics of the Prophet (peace be upon him) are mentioned.
(Note: The original wording of a hadith is called the "Matn". The chain of narrators before the Matn is called the "Isnad". If no narrator is missing from the Isnad, it is called "Mutasil" (continuous), otherwise "Munqati" (disconnected).)

The command of Allah which the Prophet (peace be upon him) has narrated from Allah, reached us through narrators, and is not present in the Qur'an.

Another definition of Hadith Qudsi: A Hadith in which the Prophet (peace be upon him) says: "Allah Ta'ala has said."

The hadith in which any statement, action, or utterance is attributed to the Messenger of Allah (peace be upon him).

The hadith in which any statement, action, or utterance is attributed to a Companion (Sahabi).

The hadith in which any statement or action is attributed to a Successor (Tabi‘i) or the Successor of a Successor (Taba‘ Tabi‘in).

The hadith which fulfills the four conditions of Mutawatir:
SB (a) EB It is narrated by a large number of narrators.
SB (b) EB Human intellect and custom consider it impossible for them to lie.
SB (c) EB This abundance exists in every generation of the chain from the era of prophethood until the time of the compiler of the book.
SB (d) EB The hadith relates to human observation or hearing.
Note: A group of narrators who have heard the hadith from one or more teachers is called a "generation" (Tabaqah).

The hadith which does not fulfill the conditions of Mutawatir.
There are four types of Ahad (single) hadith:
➊ Mashhoor (well-known)
➋ Mustafiz
➌ Aziz
➍ Gharib

The hadith in which the number of narrators in each generation is more than two but not equal, e.g., three narrators in one generation, four in another, and five in another.

SB Another definition of Mashhoor hadith: EB Mashhoor hadith is one narrated by at least three narrators in every generation. Hadiths with more than three narrators are also called Mashhoor as long as they do not reach the level of Mutawatir.

SB Warning: EB Being Mashhoor is not a proof of authenticity; sometimes false reports also become famous.

SB Second warning: EB Generally, a Mashhoor narration is understood as one known by most people, for example: "Seek knowledge even if you have to go to China."

The hadith in which narrators in every generation are more than two and equal in number, or the number of narrators in the beginning and end of the chain are equal.

The Hadith whose narrators in any generation are only two.

The Hadith narrated by only one narrator in a certain generation. If that narrator is a Sahabi (Companion) or Tabi‘i (Follower), it is called "Ghareeb Mutlaq" (Absolute Ghareeb), and if the narrator is someone else, it is called "Ghareeb Nisbi" (Relative Ghareeb).

Note SR: ER Among the above types, Mutawatir Hadith is true to the level of certain knowledge (Ilm al-Yaqin). Other types can be acceptable or rejected.

Another definition of Ghareeb Hadith: Ghareeb Hadith is one in which only one narrator remains in any generation of the chain (Sanad).

Warning: Usually people consider Ghareeb narration weak, but this is not true; Ghareeb narrations can be authentic or weak.

The Hadith that is obligatory to act upon.

The Hadith that is not acceptable.

The Hadith which fulfills the five conditions of authenticity:
SB (a) EB Its chain (Sanad) is connected, meaning every narrator received it directly from his teacher.
SB (b) EB Every narrator is just (‘Adil’), i.e., avoids major sins, does not persist in minor sins, is of good nature and morals. He is perfect in memory and precision, i.e., preserves the Hadith accurately by writing or memorization and transmits it properly.
SB (c) EB It is not a ‘Shadh’ (contradictory) Hadith.
SB (d) EB It is not ‘Ma‘lul’ (defective).
Explanation of ‘Shadh’ and ‘Ma‘lul’ will follow.

When a Hasan (good) Hadith has more than one chain of narration, it can rise from the rank of Hasan to the rank of Sahih (authentic). This is called "Sahih Lighairihi" because it attains the level of authenticity due to its other chains (other Sanads).

The Hadith whose some narrators are "light in precision" (Khafif ad-Dabt) compared to narrators of Sahih Hadith, but all other conditions remain the same.

The Hadith which has multiple chains (Sanads), each having slight weakness, but the weakness is compensated by the multiple chains, then it attains the rank of "Hasan Lighairihi" (good due to others).

The Hadith found in both Sahih Bukhari and Sahih Muslim is called "Muttafaqun Alayh" (agreed upon) and is considered at the highest level of authenticity.

Any Hadith found in Sahih Bukhari but not found in Sahih Muslim.

Any Hadith found in Sahih Muslim but not found in Sahih Bukhari.

Any Hadith not found in either Sahih Bukhari or Sahih Muslim, but is authentic according to the conditions of both Imams.

Any Hadith authentic according to Imam Bukhari’s conditions but not present in Sahih Bukhari.

Any Hadith authentic according to Imam Muslim’s conditions but not present in Sahih Muslim.

Any Hadith authentic according to the conditions of other Muhaddithin (Hadith scholars) besides Imam Bukhari and Imam Muslim.

A Hadith whose initial part or the entire chain (Sanad) is deliberately omitted.

Another definition of Muallaq: A Muallaq narration is one in which the author does not mention some narrators from the beginning of the chain but starts the chain from the middle or end; in other words, narrators from the start of the chain are omitted.

A Hadith narrated by a Tabi‘i (Follower) directly from the Prophet Muhammad (peace be upon him) without mentioning the companion (Sahabi) as an intermediary.

Another definition of Mursal: A Mursal Hadith is one in which a Tabi‘i narrates directly from the Prophet without mentioning the companion as a link.

A Hadith in which two or more narrators are omitted together from the middle of the chain.

A Hadith in which one or more narrators are omitted from different places in the middle of the chain.

A Hadith whose narrator for some reason hides the name (or identity) of his teacher or his teacher’s teacher but gives the impression to listeners that he did not do so. The chain is thus considered connected, although the meeting and hearing between narrators in the chain are established, the actual hearing of the narration is not proven.

A Hadith whose narrator narrates from a contemporary whom he has not met or whose meeting is not established.

A Hadith that appears “acceptable” on the surface but contains hidden defects or faults that make it “unacceptable.” Detecting these faults and defects is the work of experts and is not something everyone can do.

A Hadith whose narrator believes in or practices an innovation (bid‘ah) that causes disbelief (kufr), but if the narrator’s innovation is not of disbelief and he is also just (‘Adil’) and precise (Dhabit), then his narration is considered reliable. Note: Bid‘ah causing disbelief (kufr) leads to apostasy.

A Hadith whose narrator commits major sins but does not reach the level of disbelief.

A Hadith whose narrator lies in ordinary conversation and whose narration has been rejected by the Hadith scholars.

A Hadith whose narrator has lied regarding Hadith matters at some point; every narration from such a narrator is called fabricated (Mawdu‘).

A Hadith whose wording appears correct but whose pronunciation has changed due to changes in diacritics (dots), vowel marks, or pauses.

A Hadith in which, due to a narrator’s mistake, words have been rearranged or a narrator in the chain is replaced by another narrator.

A Hadith in which a narrator’s own words, intentionally or unintentionally, are inserted and the inserted words may be mistaken as part of the Hadith text.

When two narrators narrate the same chain, one being trustworthy (Thiqah) and the other more trustworthy, and the trustworthy narrator adds one narrator in the chain, then his narration is called “Mazeed fi Muttasil al-Asanid” (Addition in connected chains).

A Hadith whose narrator is acceptable (Thiqah or Saduq) but contradicts many narrators who are more or equally trustworthy; the Hadith is called “Mahfuz” in contrast to “Shadh.”

A Hadith whose narrator is weak and contradicts one or more trustworthy narrators; the Hadith is called “Maruf” in contrast to “Munkar.”

A Hadith whose narrator is “Sayyi’ al-Hifz” (born with weak memory).

A Hadith whose narrator commits severe negligence or many errors.

A Hadith whose narrator commits obvious and clear mistakes.

A Hadith whose narrator loses memory due to old age or accident, or whose written Hadiths get lost.

A Hadith whose chain or text contains differences among narrators that cannot be resolved.

A Hadith whose narrator is “Majhul al-‘Ayn” (unknown in identity), meaning no comments from scholars indicate if he is trustworthy or weak, and only one student narrates from him, thus his identity remains unknown.

A Hadith whose narrator is “Majhul al-Hal” (unknown in condition), meaning no scholarly comments exist, and only two narrators narrate from him, making his identity known but his condition unknown. Such a narrator is also called “Mastur” (concealed).

A Hadith whose chain does not explicitly mention the name of a narrator.

Meaning all narrators of the Hadith are honest and truthful; this judgment is made considering the opinions of other critics.

Meaning all narrators have strong memory.

Warning: Later scholars, recognizing differences in memory strength, coined the term “Hasan” for narrations from narrators with less than perfect precision, whereas earlier scholars did not differentiate and considered every precise narration as “Sahih.” Some early scholars used “Hasan” differently. There is a misconception that early scholars did not consider Hasan Hadith authoritative, but this is not true; they simply did not use the term “Hasan,” and used “Sahih” instead.

It means all narrators of the Hadith directly acquired it from the person they narrated from.

Any book whose author is committed to including authentic narrations and includes “Sahih” (authentic) in the title. The narrations in such a book are considered authentic at least according to the author. Even if the author points out faults in some Hadiths, it does not negate the book’s authenticity.

The six books Sahih Bukhari, Sahih Muslim, Sunan Abu Dawood, Sunan Nasai, Jami‘ at-Tirmidhi, and Sunan Ibn Majah are called “Sihah Sittah” (The Authentic Six). They are also called “Usul Sittah” or “Kutub Sittah.”

The first two books (Sahih Bukhari and Sahih Muslim) are called “Sahihain” and are not only authentic according to their authors but are held at the highest level of authenticity by the entire Ummah. According to Shah Waliullah Muhaddith Dehlawi (may Allah have mercy on him), anyone who objects to them is opposing the consensus of the Ummah and is innovating.

The last four books (Sunan Abu Dawood, Sunan Nasai, Jami‘ at-Tirmidhi, and Sunan Ibn Majah) are called “Sunan Arba‘ah.” Though they contain some weak Hadiths, due to the abundance of authentic narrations, most scholars include them in the “Sihah Sittah.”

A book in which Hadiths related to all Islamic topics such as beliefs, rulings, tafsir, Paradise, Hellfire, etc., are narrated, e.g., Sahih Bukhari and Jami‘ at-Tirmidhi.

A book in which only Hadiths related to practical rulings are collected and organized according to jurisprudential chapters, e.g., Sunan Abu Dawood.

A book in which narrations of one or multiple companions are collected separately, e.g., Musnad Ahmad, Musnad Hamidi.

A book in which the author narrates Hadiths from another book using his own chains, e.g., Mustakhraj Asma‘ili ‘ala Sahih Bukhari.

A book in which the author collects narrations that meet the conditions of another author but are not found in that author’s book, e.g., Mustadrak Hakim.

A book in which the author collects the narrations of each of his teachers separately in a specific order, for example: Mu‘jam Tabarani.

A book in which forty Hadiths are collected on one or various topics, for example: Arba‘in Nawawi, Arba‘in Thanai, etc.

A book in which narrations of only one narrator or on one subject are collected, such as Imam Bukhari’s “Juz’ Raf‘ al-Yadain” and “Juz’ al-Qira’ah Khalf al-Imam” or Imam Bayhaqi’s “Kitab al-Qira’ah Khalf al-Imam,” etc.