Umma was an ancient city in Sumer. Sumer (or Shumer, Sumeria, Shinar, native ki-en-gir) formed the southern part of Mesopotamia from the time of settlement by the Sumerians until the time of Babylonia. ...
A word describing the entire population of Muslims, that is, everyone who defines themselves as a Muslim. In the Koran it is used to describe a group of people that are a part of a divine plan and salvation. Umma is often cited by Muslim extremists as the path or the reason they engage in their struggle, as Umma would be the end result of that struggle, where everyone converts or is converted to Islam. The word "Umma" in modern Islam describes a central theological concept, and has been crucial how Muslims perceive themselves in relation to other Muslims. (We are unified by our faith.)
Umma used for a group is often to be understood as confined by ethnicity or linguistics.
It appears that in the early days of Islam, umma was used for the population of Mecca, but with the development of Islam, the umma of Muhammad changed to become believers, and therefore excluded Meccans that had not converted.
The umma term has without being a central theological concept, been crucial to the Muslim understanding of unity.