The Arabic word sunna lexically means "road" or "practice." In the language of the Prophet and the Companions it denotes the whole of licit [lawful] practices followed in the Religion (dîn), particularly the pristine (hanîf) 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 (taqrîr), or attribute (sifa), whether physical (khilqiyya) or moral (khuluqiyya) ascribed to (udîfa ila) the Prophet , whether before or after the beginning of his prophethood.1 Thus the "sciences of the Sunna" (`ulûm al-Sunna) refer to the biography of the Prophet (al-sîra), the chronicle of his battles (al-maghâzî), his everyday sayings and acts or "ways" (sunan), his personal and moral qualities (al-shamâ'il), and the host of the ancillary  hadîth sciences such as the circumstances of occurrence (asbâb al-wurûd), knowledge of the abrogating and abrogated hadîth, difficult words (gharîb al-hadîth), narrator criticism (al-jarh wal-ta`dîl), narrator biographies (al-rijâl), etc., as discussed in great detail in the authoritative books of al-Khatîb al-Baghdâdî.
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.
Imâm Ahmad said: "The Sunna in our definition consists in the reports transmitted from the Messenger of Allah , and the Sunna is the commentary (tafsîr) of the Qur'ân and contains its directions (dalâ'il)."
The early Sunnî Masters such as Abû Hanîfa, al-Humaydî, Ibn Abî `Âsim, Abû Dâwûd, and Abû Nasr al-Marwazî also used the term "the Sunna" in the narrow sense to refer to Sunnî Doctrine as opposed to the creeds of non-Sunnî sects.
In the terminology of usul al-fiqh or principles of jurisprudence, sunna denotes a saying (qawl), action (fi`l) or approval (taqrîr) related from (nuqila `an) the Prophet 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 (dalîl shar`î) but without being obligatory, the continued abandonment of which constitutes disregard (istikhfâf) of the Religion - also sin (ithm) according to some jurists - and incurs blame (lawm, `itab, tadlîl) - also punishment (`uqûba) 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 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-`âda).
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 fî al-sunna al-wujûb lâ al-ibâha). Sunna is thus defined as the strongest of the following near-synonymous categories:"praiseworthy" (mandûb)
Al-Dhahabî relates from Ishaq ibn Rahuyah the saying: "If al-Thawri, al-Awza`i, and Mâlik concur on a given matter, that matter is a Sunna." Al-Dhahabî comments: