Ijma’ (or Consensus) is the term used for an opinion or command of Islam where all the good and respected scholars of Islam are unanimous in their ruling. It is in the third position after Qur’an and Sunnah.
For example, there is Ijma’ amongst the scholars that there are five obligatory prayers, or that adhaan must be given before the prayers, or that a funeral prayer must be read over a deceased believer, or that swine is prohibited in Islam, etc. 
In Arabic (إجماع) the term Ijma' has two meanings:
- Determination and resolve. أجمع/Ajma’ to become determined. The Prophet ﷺsays: "He will not be fasting [the next morning] who does not resolve to do so by night". (Sunan an-Nasa'i 2342). Ijma' can also mean one person's intent or resolution.
- Agreement or consensus: Of a people, أجمعو/Ajma ' u refers to their beingof the same opinion whether in secular or religious matters.
As far as legal theorists are concerned, ijma' occurs when scholars of the Muslim nation following the Prophet's death, are in agreement about a case of Shari'ah, a term which refers here to whatever is included in a law-maker's pronouncements.
Proof of Ijma’ in Qur’an
‘And whoever opposes the Messenger after guidance has become clear to him and follows other than the way of the believers - We will give him what he has taken and drive him into Hell, and evil it is as a destination.’ (Surah An Nisa 4:115). Thus, whosoever follows a path other than that of the believers deserves to be punished in Hell. That also means that the believers' path must be the correct one.
Proof of Ijma’ in Hadeeth
The Prophet's hadith: " My Ummah will not unite upon error(misguidance).” Sahih al Jam’ee 1786
Types of Ijma’
There exist two types of Ijma':
1.Evident/ Definitive Ijma':
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. 
2. Silent ijma':
Here, a scholar renders an opinion which is communicated to other scholars who neither openly support it nor openly oppose or differ it, provided they are free from fear of persecution or other such fears. Here also, scholars should be allowed ample time to understand the issue under judgment, but he keeps silent. 
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.
Conditions of ijma‘
There are certain conditions for ijmaa‘, such as:
- 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.
- 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. 
Examples of Ijma’
- Share of Grandmother being 1/6th from the grandson’s wealth
- Ijma' has also decided against a man's marrying his wife's aunt, niece or sister, and construes a step-brother and a step-sister on the father's side as blood brothers and sisters, in the absence of brothers and sisters.
- According to Ijma', a Muslim woman does not marry a non-Muslim, if she does the marriage is deemed null and void.
- Muslim consensus over choosing Abu Bakr as Caliph after the Prophet's death, over fighting the refiners of zakat and over compiling the Munificent Qur'an in the mushaf are all instances of Ijma'.
- Another such instance is forbidding the distribution of open lands to Muslim conquerors as spoils of war, and endowing them as "Waqf' in favor of Muslims, from the proceeds of which judges, workers and soldiers could be paid, and widows and orphans provided for. After the conquest of Syria (Ash-Sham) and Iraq during the Caliphate of 'Umar Ibn al Khhattab, some companions were against this ruling such as Abdul Rahman Ib Awf and `Ammar Ibn Yasir. Meanwhile, 'Umar Ibn al Khattab, `Ali Ibn Abu Talib and 'Uthman Ibn `Affàn were, all in favor. Opposers objected that `Umar was giving ordinary Muslims that which the conquerors earned by the work of their sword. 'Umar hypothesized over the situation where future generations of Muslims would be in to find the land divided and inherited. 'Umar pursued his argument until his views were approved by the majority. An Ijma' was thus established.
Few believed ijma’ is not a proof and based their argument on the following:
- O you who have believed, obey Allah and obey the Messenger and those in authority among you. And if you disagree over anything, refer it to Allah and the Messenger, if you should believe in Allah and the Last Day. That is the best [way] and best in result. (Surah Nisa 4:59)
- When the Messenger of Allah (ﷺ) intended to send Mu'adh ibn Jabal(R) to the Yemen, he asked: How will you judge when the occasion of deciding a case arises?
He replied: I shall judge in accordance with Allah's Book. He asked: (What will you do) if you do not find any guidance in Allah's Book? He replied: (I shall act) in accordance with the Sunnah of the Messenger of Allah (ﷺ).
He asked: (What will you do) if you do not find any guidance in the Sunnah of the Messenger of Allah (ﷺ) and in Allah's Book?
He replied: I shall do my best to form an opinion and I shall spare no effort.
The Messenger of Allah (ﷺ) then patted him on the breast and said: Praise be to Allah Who has helped the messenger of the Messenger of Allah to find something which pleases the Messenger of Allah. Sunan Abi Dawud 3592, Da'if Al-Albani
The arguments used here are weak. The ayath does not rule out ijma'. What it does, however, is enjoin obedience to Allah and the Prophet ﷺ.It refers judgment over disputes to the Almighty, His Messenger and those in charge. ijma' does refer to the Book and to Sunnah in making judgments, which renders the first argument invalid. As to the hadith, Ibn Jabal could not mention ijma', because it simply was unthinkable during the Prophet's lifetime. Moreover that hadith is Da’if according to Shaikh Albani. 
Ijtihad; Nafl; Taqlid; Sahaba; Sunnah;
  http://islamicstudies.islammessage.com/panel/media/file/Philosophy-of-Islamic-Shariah.pdf