Ḥadīth = “the corpus of traditions from the Prophet”; reports of words and sayings attributed to the Prophet Muḥammad (for Sunnīs) as well as to a series of Imams who succeeded him (for Shīʿa); the text of the second source of Islamic law.
Muḥammad is undoubtedly one of the central figures in Islamic law and authority. His legal authority emanates from two primary phenomena. First, he is the transmitter of God’s divine revelation, that is, the Qurʾān. Therefore, he is the immediate, although not ultimate, source of divine commands. Second, a complex set of prophetical sayings and teachings is attributed to the Prophet (ḥadīth). Though inspired by divine will, these sayings constitute Muhammad’s direct contribution to Islamic law, consolidating his centrality to Islamic authority.
Sunna, as the second source of Islamic law, is defined almost entirely by the ḥadīth corpus of texts. The term ḥadīth has come to denote various things over centuries of developments within Islamic tradition. In the early days of Islamic thought, some regarded sunna, which is “an instance of normative behavior, that is, a practice or precedent to be emulated,” to be a concept much larger than ḥadīth. Sunna, in the early days of Islamic thought, did not only denote the Prophet’s practice and sayings. Instead, it meant, more generally, the way of living during the Prophetic era. The equating of ḥadīth with Prophetic sayings was a later development, led by two founding jurists of Islamic law, Shāfi‘ī and Ibn Ḥanbal.
Some scholars advocated ḥadīth as a complement to the Qurʾānic text. These ḥadīth -advocates came to be known as ahl al-ḥadīth [proponents of ḥadīth -texts]. Their opponents, who advocated for the exercise of interpretation through ordinary legal reasoning wherever the Qurʾān was ambiguous, were called ahl al-kalām or ahl al-ra’y [proponents of interpretation].
Tens of thousands of these ḥadīths, as sayings attributed to the Prophet over a long period of time and space, were collected under many books. Among Sunnīs, six of these volumes were later canonized within the tradition bearing the following titles to signify that earlier scholars regarded them as reliable compilations: sahīh, sunan, musnad, and musannaf. Each of these terms meant, respectively, authentic, using the plural of sunna, supported by isnad (a chain of transmitter of the ḥadīth), and compilation. The Sahīhs of Bukhārī and Muslim are regarded as the most authentic and authoritative ḥadīth compendia to this date in Sunnī law. [In Shīʿī law, a different set of four books were regarded as reliable.] However, for both, the sense of reliability has since changed for some modern critical scholars.
The canonization of these books was followed by the emergence of ḥadīth criticism as a separate discipline in Islamic studies (‘ilm [or ‘ulūm] al-ḥadīth).” This distinct field of study, among other things, focused on the veracity and matters such as theology of the transmitters, by which critics variously rejected and approved them, through a process called jarh wa-ta‘dīl. The field also categorized ḥadīths on the basis of relative reliability or lack thereof, “ranging from the absolutely trustworthy to the thoroughly unreliable”: sahīh (sound, authentic), hasan (good, acceptable), da‘īf (weak), shādhdh (having an unusual text), mawdū‘ (forged). Accompanying this field was a related discipline in which ḥadīth critics recorded the biographies of ḥadīth narrators to assess aspects of their lives that would aid in determining the veracity of their statements. These biographical dictionaries focused on the narrators' “probity, scholarly ability, and relevant biographical details.”
Reports with “multiple lines of transmission” were called mutawātir, which signified a high standard of reliability that applied to only a few ḥadīths. Others were labeled either single-source transmissions (akhbār āhād), which was the weakest standard of reliability, or well-known transmissions (mashhūr), which was an intermediate standard. There were some ḥadīths that signified the same meaning through multiple transmissions, although their texts differed from one another. This phenomenon led scholars to develop a concept of conceptual widespread transmission (tawātur ma‘nawī), effectively adding this set of ḥadīths to the list of mutawātir ḥadīths with the highest standard. Finally a separate set of non-Prophetic reports of Muḥammad’s companions and their successors were also a type of ḥadīth, called athar, and this set of statements also found their way into ḥadīth compilations.
This article provides a summary of definitions on ḥadīth from Joseph E. Lowry, "The Prophet as Lawgiver and Legal Authority," in The Cambridge Companion to Muhammad (Jonathan E. Brockopp ed., 2010), 83-102.