Dought&clear, - Ijmaa' (scholarly consensus): definition, types and conditions

Praise be to Allah.
Shaykh Ibn 'Uthaymeen (may Allah have mercy on him) said:
In linguistic terms, ijmaa' means resolve and agreement.
In shar'i terms, it means the agreement of the mujtahids of this ummah
after the death of the Prophet (blessings and peace of Allah be upon
him) on a shar'i ruling.
By saying "agreement" we exclude differences of opinion; if there is a
difference of opinion, even from one person, then we cannot say that
there is ijmaa'.
By saying "the mujtahids" we exclude the common folk and those who
follow or imitate scholars; it does not matter whether they agree or
disagree.
By saying "this ummah" we exclude the consensus of others, which
carries no weight.
By saying "after the death of the Prophet (blessings and peace of
Allah be upon him)" we exclude their agreement at the time of the
Prophet (blessings and peace of Allah be upon him); ijmaa' or
consensus at that time does not count as evidence, because evidence is
established by the Sunnah of the Prophet (blessings and peace of Allah
be upon him), whether in word or deed or by approval. Hence if a
Sahaabi says "We used to do" or that they (i.e., people) used to do
such and such at the time of the Prophet (blessings and peace of Allah
be upon him), this is indicative of the approval of the Prophet
(blessings and peace of Allah be upon him), according to consensus.
By saying "on a shar'i ruling", we exclude their agreement on a
rational or human ruling, which has nothing to do with the matter
under discussion, because we are talking about looking for ijmaa' as
one of the kinds of shar'i evidence.
Ijmaa' counts as evidence on the basis of a number of pieces of
evidence, including the following:
1. The verse in which Allah, may He be exalted, says
(interpretation of the meaning):"Thus We have made you (Muslims), a
Wasat (just) (and the best) nation, that you be witnesses over
mankind" [al-Baqarah 2:143]. The words "witnesses over mankind"
include testifying about their deeds and judging their deeds, and the
words of the witness may be accepted.
2. The verse in which Allah, says (interpretation of the
meaning):"(And) if you differ in anything amongst yourselves"
[an-Nisa'4:59]indicate that whatever they agreed upon is sound and
correct.
3. The Prophet (blessings and peace of Allah be upon him) said:
"My ummah will not unanimously agree on misguidance."
4. We say: If the ummah unanimously agrees on something, it must
be either true or false. If it is true, then it is proof. If it is
false, how can this ummah, which is the dearest of nations to Allah
since the time of its Prophet until the onset of the Hour, agree on
something false with which Allah is not pleased? This is quite
impossible.
Types of ijmaa'
Ijmaa' is of two types: definitive and presumptive.
1. Definitive is that which well known and well established, such
as consensus that the five daily prayers are obligatory and that zina
(fornication, adultery) is haraam. No one can deny that this type of
ijmaa' is proven and established, or that it constitutes proof in and
of itself, or that the one who rejects it becomes a kaafir, unless he
is ignorant and may be excused for his ignorance.
2. Presumptive is that which can only be known by means of
research and study, where the scholars may differ as to whether is
ijmaa' (on a particular issue) or not. The most correct scholarly
opinion concerning that is the view of Shaykh al-Islam Ibn Taymiyah,
when he said inal-'Aqeedah al-Waasitiyyah: The type of ijmaa' that is
to be accepted is that of the righteous early generations (as-salaf
as-saalih), because after their time there was a great deal of
disagreement and the ummah spread far and wide. End quote.
It should be noted that the ummah cannot agree on something that is
contrary to an unabrogated, clear, saheeh text, because it can only
agree on what is true. If you see consensus that you think is contrary
to that, then it must be one of the following: either the evidence is
not clear, or it is not saheeh, or it is abrogated, or there is a
difference of opinion concerning the matter of which you were not
aware.
Conditions of ijmaa':
There are certain conditions for ijmaa', such as:
1. It should be soundly proven in the sense that it is either well
known among the scholars or transmitted by a trustworthy narrator who
has read widely.
2. It should not have been preceded by a well-known difference of
opinion. If that was the case, then there is no ijmaa', because
scholarly opinions are not invalidated by the death of their authors.
Ijmaa' does not cancel out a previous difference of opinion; rather it
prevents differences of opinion from arising.
This is the most correct view, because of the strength of its argument.
And it was said that the second condition is not stipulated, so it is
valid in a later period for there to be consensus on one of the
previous opinions and for that to serve as proof for those who come
afterwards.
According to the majority, it is not essential that those who
unanimously agree all die when still holding this view for ijmaa' to
be established; rather ijmaa' is established as soon as they (the
scholars of a particular era) agree, and it is not permissible for
them or anyone else to go against it after that, because the condition
for the establishment of ijmaa' do not include any stipulation that
the era (of the scholars who reached this consensus) should have come
to an end with their passing. As ijmaa' is established at the moment
they agree (on a particular issue), there is nothing that could cancel
it out.
If one of the mujtahids (scholars) says or does something and that
becomes well known among the mujtahids, and they do not denounce it
even though they are able to do so, then it is said that there is
ijmaa'. It was said that this establishes that there is ijmaa'; others
said that it is may be regarded as proof but not ijmaa'; and others
said that it is neither ijmaa' nor proof. And it was said that if they
all passed away before denouncing it then it is ijmaa', because their
silence until the time of their death, even though they were able to
denounce it, constitutes proof of their agreement. This is the view
that is most likely to be correct.
Al-Usool min 'Ilm al-Usool, 62-64
And Allah knows best.