The Arabic word sunna lexically means "road" or "practice." In the language of the Prophet ﷺ and the Companions it denotes the whole of licit [lawful] practices followed in the Religion (dīn), particularly the pristine (hanīf) path of Prophets, whether pertaining to belief, religious and social practice, or ethics generally speaking.
In its technical sense sunna has three meanings. In hadith terminology it denotes any saying (qawl), action (fi'l), approval (taqrīr), or attribute (sifa), whether physical (khilqiyya) or moral (khuluqiyya) ascribed to (udīfa ila) the Prophet ﷺ, whether before or after the beginning of his prophethood.1 Thus the "sciences of the Sunna" (ʿulūm al-Sunna) refer to the biography of the Prophet ﷺ (al-sīra), the chronicle of his battles (al-maghāzī), his everyday sayings and acts or "ways" (sunan), his personal and moral qualities (al-shamā'il), and the host of the ancillary  hadīth sciences such as the circumstances of occurrence (asbāb al-wurūd), knowledge of the abrogating and abrogated hadīth, difficult words (gharīb al-hadīth), narrator criticism (al-jarh wal-taʿdīl), narrator biographies (al-rijāl), etc., as discussed in great detail in the authoritative books of al-Khatīb al-Baghdādī.
This meaning is used in contradistinction to the Qur'an in expressions such as "Qur'an and Sunna" and applies in the usage of hadith scholars.
Imām Ahmad said: "The Sunna in our definition consists in the reports transmitted from the Messenger of Allah ﷺ, and the Sunna is the commentary (tafsīr) of the Qur'ān and contains its directions (dalā'il)."
The early Sunnī Masters such as Abū Hanīfa, al-Humaydī, Ibn Abī ʿāsim, Abū Dāwūd, and Abū Nasr al-Marwazī also used the term "the Sunna" in the narrow sense to refer to Sunnī Doctrine as opposed to the creeds of non-Sunnī sects.
In the terminology of usul al-fiqh or principles of jurisprudence, sunna denotes a saying (qawl), action (fiʿl) or approval (taqrīr) related from (nuqila ʿan) the Prophet ﷺ or issuing (sadara) from him other than the Qur'an.
In the terminology of fiqh or jurisprudence, sunna denotes whatever is firmly established (thabata) as called for (matlub) in the Religion on the basis of a legal proof (dalīl sharʿī) but without being obligatory, the continued abandonment of which constitutes disregard (istikhfāf) of the Religion - also sin (ithm) according to some jurists - and incurs blame (lawm, ʿitab, tadlīl) - also punishment (ʿuqūba) according to some jurists.2 However, some jurists have made a distinction between what they called "Emphasized Sunna" (Sunna mu'akkada) or "Sunna of Guidance" (Sunna al-huda), such as what the Prophet ﷺ ordered or emphasized in word or in deed, and other types of Sunna considered less binding in their legal status, such as what they called "Non-Emphasized Sunna" (Sunna ghayr mu'akkada) or "Sunna of Habit" (Sunna al-ʿāda).
The above jurisprudential meanings of Sunna are used in contradistinction to the other four of the five legal categories for human actions - fard (obligatory), sunna, mubah (indifferent), makruh (disliked), haram (prohibited) - and applies in the usage of jurists from the second Hijri century onwards. However, the jurists have stressed that the basis for all acts of worship categorized as sunna is "obligatoriness" not "permissiveness" (al-asl fī al-sunna al-wujūb lā al-ibāha). Sunna is thus defined as the strongest of the following near-synonymous categories:"praiseworthy" (mandūb)
Al-Dhahabī relates from Ishaq ibn Rahuyah the saying: "If al-Thawri, al-Awzaʿi, and Mālik concur on a given matter, that matter is a Sunna." Al-Dhahabī comments: