The Collaborative International Dictionary of English v.0.48:

Fosse \Fosse\, n. [F., fr. L. fossa, fr. fodere, fossum, to
   dig.]
   1. (Fort.) A ditch or moat.
      [1913 Webster]

   2. (Anat.) See Fossa.
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   Fosse road. See Fosseway.
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The Collaborative International Dictionary of English v.0.48:

Ditch \Ditch\ (?; 224), n.; pl. Ditches. [OE. dich, orig. the
   same word as dik. See Dike.]
   1. A trench made in the earth by digging, particularly a
      trench for draining wet land, for guarding or fencing
      inclosures, or for preventing an approach to a town or
      fortress. In the latter sense, it is called also a moat
      or a fosse.
      [1913 Webster]

   2. Any long, narrow receptacle for water on the surface of
      the earth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fosse
    n 1: ditch dug as a fortification and usually filled with water
         [syn: moat, fosse]