A religious denomination, (also: denomination) is a large, long-established subgroup within a religion that has been in existence for many years.
The term is frequently used to describe the different Christian churches (Orthodox, Catholic and the many varieties of Protestantism); it is also used to describe the three main branches of Judaism (Orthodox Judaism, Conservative Judaism and Reform Judaism), and (less often, though it would not be inappropriate) to describe the two main branches of Islam (Sunni and Shia).
Denominations usually have a significant degree of authority over their member congregations, although the term is also used to describe religious groups when the congregations have authority over the 'denomination', such as the numerous Baptist associations or the Unitarian Universalist Association.
Denominations often form slowly over time for many reasons; due to historical accidents of geography, culture, and influence between different groups, members of a given religion slowly begin to diverge in their views. Over time members of a religion may find that they have developed significantly different views on theology, philosophy, religious pluralism, ethics and religious practices and rituals. As such, in any of a myriad of ways, different denominations eventually form. In other cases, denominations form very rapidly, either as a result from a split or schism in an existing denomination, or as people from many different denominations share an experience of spiritual revival or awakening, and choose to form a new denomination based on that new experience or understanding.
An example within Christianity is the Mennonite and the Church of the Brethren denominations. Both denominations are similar in their beliefs, yet they are unique because they were started by a different person, (Menno Simons and Alexander Mack respectively). Their division is administrative, and there is much communication and interaction between the two. Since its founding, the Mennonite denomination has split into a number of smaller Mennonite denominations, because of both geography and social and theological differences.
Another example is the Roman Catholic Church and various Protestant churches such as the Lutheran Churches. When Martin Luther founded the Lutheran Church, he and his followers were persecuted and sometimes killed as heretics. The early Lutherans in turn persecuted and sometimes killed the Anabaptists as heretics. Even today there are major ideological differences between them, even though there is no physical hostility.