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"