The Collaborative International Dictionary of English v.0.48:

Diocese \Di"o*cese\, n.; pl. Dioceses. [OE. diocise, OF.
   diocise, F. dioc['e]se, L. dioecesis, fr. Gr. ? housekeeping,
   administration, a province, a diocese, fr. ? to keep house,
   manage; dia` through + ? to manage a household, ? a house.
   See Economy.] (Eccl.)
   The circuit or extent of a bishop's jurisdiction; the
   district in which a bishop exercises his ecclesiastical
   authority. [Frequently, but improperly, spelt diocess.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

diocese
    n 1: the territorial jurisdiction of a bishop [syn: diocese,
         bishopric, episcopate]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "diocese":
   Kreis, archbishopric, archdiocese, arrondissement, bailiwick,
   bishopric, borough, canton, city, commune, conference,
   congressional district, constablewick, county, departement,
   district, duchy, electoral district, electorate, government,
   hamlet, hundred, magistracy, metropolis, metropolitan area, oblast,
   okrug, parish, precinct, principality, province, region, riding,
   see, sheriffalty, sheriffwick, shire, shrievalty, soke, stake,
   state, synod, territory, town, township, village, wapentake,
   ward



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DIOCESE, eccl. law. The district over which a bishop exercises his spiritual
functions. 1 B1. Com. 111.