The Collaborative International Dictionary of English v.0.48:

Expanse \Ex*panse"\, v. t.
   To expand. [Obs.]
   [1913 Webster]

         That lies expansed unto the eyes of all. --Sir. T.
                                                  Browne.
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The Collaborative International Dictionary of English v.0.48:

Expanse \Ex*panse"\, n. [From L. expansus, p. p. of expandere.
   See Expand.]
   That which is expanded or spread out; a wide extent of space
   or body; especially, the arch of the sky. "The green
   expanse." --Savage.
   [1913 Webster]

         Lights . . . high in the expanse of heaven. --Milton.
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         The smooth expanse of crystal lakes.     --Pope.
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WordNet (r) 3.0 (2006):

expanse
    n 1: a wide scope; "the sweep of the plains" [syn: sweep,
         expanse]
    2: the extent of a 2-dimensional surface enclosed within a
       boundary; "the area of a rectangle"; "it was about 500 square
       feet in area" [syn: area, expanse, surface area]
    3: a wide and open space or area as of surface or land or sky