the territorial jurisdiction of a bishop
"The new cathedral stands at the heart of the diocese, serving as the central hub for all bishops within that territorial jurisdiction."
Administrative division of the later Roman Empire, starting with the tetrarchy.
"The early Christian hierarchy adopted the existing administrative structure of the diocese as its own territorial unit."
In plain English: A diocese is an area of land that contains many churches and is led by a single bishop.
"The bishop serves as the spiritual leader of his local diocese."
Usage: In modern contexts, this term almost exclusively refers to an administrative district under the jurisdiction of a bishop in the Christian Church rather than its historical imperial meaning. Avoid using "diocese" when referring to secular government regions or non-religious organizational divisions.
The word "diocese" entered Middle English via the Old French diocese and ultimately traces back to Late Latin. It originally referred to a district governed by an official before evolving into its current ecclesiastical meaning in Christian tradition.