Consensus (ijmāʿ)
It is the unanimous agreement of the qualified jurist-scholars (mujtahids) of the community of Muhammad ﷺ, after his passing, on any legal matter—whether expressed in word or in deed—during any historical period.

Some leading imams have argued that real consensus could only have been achieved in the time of the Companions, for they were few in number and lived within a single, limited region. After that generation the jurists and guardians of the Sunnah became dispersed across many lands—such as Iraq, Syria, North Africa, the Hijaz, and Yemen—so that discovering the views of every jurist living in one era amid such dispersion is virtually impossible.

The majority of jurists, however, maintain that consensus is possible in every age. Thus, if an incident occurs and a mujtahid issues a fatwa about it, or a mujtahid-judge delivers a verdict, and that ruling is then transmitted among other mujtahids—whether muftis or judges—who approve it, act upon it, and no dissent is reported from anyone to whom it has reached, this constitutes a spoken consensus; the silent approval of those who keep their peace is counted as a tacit consensus. Consensus may also be practical, as when the people uniformly act in accordance with an established custom or usage.

The authority of consensus
Most scholars of uṣūl regard consensus as definitive proof and one of the fundamental sources of legislation. If the majority of mujtahids concur on the ruling of a discretionary legal issue and only a few scholars disagree, the view of the majority is not deemed consensus, yet it is still recognised as a valid legal proof because of its considerable weight.

Many jurists, sometimes in defence of their own schools or a position they favour, overstate claims of consensus; in almost any question for which they report consensus, a careful researcher will uncover disagreement.

Ibn al-Qayyim said: “Ibn al-Mundhir’s habit is that whenever he sees that most of the scholars have spoken on an issue…”