Excommunication literally means 'out of communion' and is the removal of a member from standing in a church body. Most excommunications are automatic under church doctrine, as in the automatic excommunication for facilitating abortion.
Priests can be automatically excommunicated. If, for example, a priest ever reveals the contents of a confession, then he is automatically excommunicated.
Excommunication is a drastic step that may result in cutting ties with other members of that church as well. In the Roman Catholic Church, to be excommunicated mainly results in being prohibited from receiving the sacraments (but not removed from the jurisdiction of the Church). If certain conditions are met, the order may later be lifted.
The concept of excommunication is discussed in the New Testament and was made famous during the Middle Ages and Renaissance periods in references to the Catholic Church. It continues to be practised into the 21st century. It is also utilized by churches such as the Conservative Mennonites, Amish, and Jehovah's Witnesses. The former two practice 'shunning' and the Witnesses refer to their practice as 'disfellowshipping'.
Recognition of an excommunication can occur even after someone has passed away.
Excommunications by Pope Francis
Though portrayed as liberal by the media, Pope Francis excommunicated an Australian priest who was pro-same-sex marriage and favored allowing women clergy. The excommunication was in Latin and reportedly did not provide an explicit reason, but the bases were well-understood.
Excommunication for pacifism
At the Council of Arles in A.D. 314, which was the first conclave of Christian bishops in the Roman Empire in the West, excommunication was established for pacifism, for those “who throw away their weapons in time of peace."
- Watchtower Bible and Tract Society. "Discipline That Can Yield Peacable Fruit" in The Watchtower; (April 15, 1988)