What is meant by the saying, "The Ummah has unanimously agreed upon something," and does this refer only to the consensus of the Ahl al-Sunnah wal-Jama'ah, or does it include other sects such as the Khawarij?

Ijma' (consensus) is the agreement of all the mujtahid (independent jurist) scholars of the Muslim nation after the death of the Messenger of Allah, peace and blessings be upon him, on a legal ruling in any era. It is stipulated that the agreement must be from the mujtahids only, thereby excluding those who merely follow (muqallidoon), disbelievers, and proponents of disbelieving innovations.