Plural of diocese; multiple geographic areas each under the authority of a bishop in the Christian church.
From Medieval Latin 'diocesis,' plural 'dioceses,' from Greek 'diokesis' (administration), used for church administrative divisions from the 4th century CE forward.
The plural 'dioceses' keeps the Greek ending rather than becoming 'dioceses' with an English '-es'—it's a Latinate plural that English reluctantly inherited because the church's vocabulary was always formal and scholarly, resistant to everyday simplification.
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