The word diocesan means relating or pertaining to a diocese. It can also be used as a noun meaning the bishop who has the principal supervision of a diocese. A diocese also may be referred to as a bishopric or episcopal see, though strictly the term episcopal see refers to the domain of ecclesiastical authority officially held by the bishop, and the term bishopric to the post of being bishop.
An archdiocese (or archiepiscopal see or archbishopric) is more significant than a diocese. An archdiocese is presided over by an archbishop whose see may have or have had importance due to size or historical significance. The archbishop may have metropolitan authority over any other suffragan bishops and their dioceses within his ecclesiastical province.
Especially in the Middle Ages, some bishops (e.g. prince-bishops) held political as well as religious authority within their dioceses, which in practice were thus independent or semi-independent states. Two vestiges of this remain: the Catholic Pope is, ex officio, the monarch of the State of Vatican City and the (Catholic) Bishop of Urgell, Spain, is, ex officio, the Spanish Co-Prince of the Co-Principality of Andorra, which is located entirely within his diocese. (The French Co-Prince of the Co-Principality of Andorra is, ex officio, the President of France.)
In the later organization of the Roman Empire, the increasingly subdivided provinces were administratively associated in a larger unit, the diocese (Latin dioecesis, from the Greek term διοίκησις, meaning "administration").