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and his rightly-guided successors held of doctrines, deeds, and sayings. This is the perfect and complete Sunna. That is why the Salaf of old refrained from applying the name of Sunna to anything that fell short of this."
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 [1] 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:
made Sunna, and the rightly-guided Caliphs after him. As for
Consensus (ijma`), it is whatever the ulama of the Community both
early and late have unanimously agreed upon, through either assumed
(zannî) or tacit (sukutî) agreement. Whoever deviates from such
consensus among the Successors or their successors, it is tolerated
for him alone. As for those who deviate from the three above-named
imams, then such is not named a deviation from Consensus, nor from
the Sunna. All that Ishaq meant was that if they concur on a given
matter then it is most probably correct, just as we say, today, that
it is nearly impossible to find the truth outside of what the Four
Imams of scholarly endeavor agreed upon. We say this at the same time
as we admit that their agreement on a given matter does not dictate
the consensus of the Community, but we refrain from asserting, in
relation to a matter upon which they all agreed, that the correct
position is otherwise.3 and adhere to his Sunna. Allah (swt) said: { Verily in the Messenger of Allah you have an good exemplar for him who hopes in Allah and the last Day and remembers Allah much.} Sura 33:21"
The excellent books 'Sunna Notes' Volumes 1 and 2 
vs.2.3