What is Ijma (consensus), and what are its types and conditions?

Linguistically, Ijma' (consensus) means determination and agreement. Technically, it is the agreement of the Mujtahidoon (qualified scholars capable of independent juristic reasoning) of this Ummah (Muslim nation) after the Prophet, peace and blessings be upon him, on a Shar'i (legal) ruling.

Its conditions include: that it is authentically established, that it is not preceded by a settled disagreement, and that it comes from the people of Ijtihad. It is considered a legal proof based on evidence from the Qur'an and Sunnah. According to the majority opinion, the passing of the era of those who reached the consensus is not a condition.

Ijma' is divided into definitive (qat'i) and speculative (dhanni). Definitive Ijma' is what is known to have occurred by the Ummah by necessity. Speculative Ijma' is what is known only through tracking and induction. The most preponderant view is that the regulated Ijma' is what the Righteous Predecessors (Salaf as-Salih) were upon.

It is impossible for the Ummah to agree upon something that contradicts an authentic, clear, and un-abrogated evidence.

Ijma' can also be silent (sukooti), which is what became known among the people of Ijtihad and they did not object to it despite their ability to object. The closest opinion is that it is a consensus if they passed away before objecting.