The Collaborative International Dictionary of English v.0.48:

Department \De*part"ment\, n. [F. d['e]partement, fr.
   d['e]partir. See Depart, v. i.]
   1. Act of departing; departure. [Obs.]
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            Sudden departments from one extreme to another.
                                                  --Wotton.
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   2. A part, portion, or subdivision.
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   3. A distinct course of life, action, study, or the like;
      appointed sphere or walk; province.
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            Superior to Pope in Pope's own peculiar department
            of literature.                        --Macaulay.
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   4. Subdivision of business or official duty; especially, one
      of the principal divisions of executive government; as,
      the treasury department; the war department; also, in a
      university, one of the divisions of instruction; as, the
      medical department; the department of physics.
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   5. A territorial division; a district; esp., in France, one
      of the districts composed of several arrondissements into
      which the country is divided for governmental purposes;
      as, the Department of the Loire.
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   6. A military subdivision of a country; as, the Department of
      the Potomac.
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WordNet (r) 3.0 (2006):

department
    n 1: a specialized division of a large organization; "you'll
         find it in the hardware department"; "she got a job in the
         historical section of the Treasury" [syn: department,
         section]
    2: the territorial and administrative division of some countries
       (such as France)
    3: a specialized sphere of knowledge; "baking is not my
       department"; "his work established a new department of
       literature"

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "department":
   airspace, ambit, area, arena, bailiwick, beat, belt, border,
   borderland, branch, bureau, circle, circuit, commissariat, concern,
   confines, constablery, constablewick, constabulary,
   continental shelf, control, corridor, country, demesne, discipline,
   district, division, domain, dominion, environs, field, ground,
   heartland, hemisphere, hinterland, judicial circuit, jurisdiction,
   land, march, milieu, ministry, municipality, neighborhood, office,
   offshore rights, orb, orbit, pale, part, parts, place, precinct,
   precincts, premises, province, purlieus, quarter, realm, region,
   responsibility, round, salient, secretariat, section, segment,
   sheriffalty, sheriffwick, shrievalty, soil, space, sphere,
   subdiscipline, subdivision, terrain, territory, three-mile limit,
   twelve-mile limit, unit, vicinage, vicinity, walk, worry, zone



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

DEPARTMENT. A portion of a country. In France, the country is divided into
departments, which are somewhat similar to the counties in this country. The
United States have been divided into military departments, including certain
portions of the country. 1 Pet. 293.
     2. By department is also meant the division of authority, as, the
department of state, of the navy, &c.