dioecesis

Dioecesis, dioecesis, f., third declension, a Greek loan from dioikesis, "administration, housekeeping", built on oikos, "house". Being Greek it keeps some Greek forms: accusative singular dioecesim or dioecesin, nominative plural dioeceses.

It was a secular word first. Under Diocletian a dioecesis was a bundle of provinces run as one administrative block, and only afterwards did the church take the term over for the territory of a bishop. The corpus here spells it diocesis, without the diphthong, which is the standard medieval simplification of oe and ae - the same habit that gives prelium for proelium and celum for caelum. English diocese and diocesan follow the Latin unchanged in sense.