OED Definition:
The global Islamic community, founded by Prophet Muhammad, comprising individuals bound to one another by religious ties.
In the modern-day configuration of nation states, the concept of the Ummah has arguably been supplanted by other forms of group loyalty, but it is still often invoked as a foundational idea in Islam. It is also, however, liable to be misused by those who wish to claim that all Muslims think alike and that the Ummah is a homogenous entity. This sort of usage should be avoided.
Ummah (Arabic: community, tribe, people) refers to the concept that all Muslims, irrespective of race, gender, social class, financial status or nationality, are bound together by virtue of their faith.
The concept was developed during the Medinan period (622-629 CE), a period in which the community was established in political terms by affirming social solidarity among the immigrants from Mecca (Muhajiroun) and the clans of Medina (Ansar).
Since then, there have been a number of developments that have impacted on the idea of an Ummah, most notably the Shia-Sunni schism which created a theological division in Islam.