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.
      [1913 Webster]

   Fosse road. See Fosseway.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "fosse":
   abri, approach trench, aqueduct, bunker, canal, canalization,
   channel, communication trench, countermine, coupure, crimp, cut,
   dike, ditch, double sap, dugout, entrenchment, fire trench,
   flying sap, fortified tunnel, foxhole, gallery, goffer, gutter,
   ha-ha, kennel, mine, moat, parallel, pleat, sap, slit trench,
   sunk fence, trench, trough, tunnel