The Collaborative International Dictionary of English v.0.48:

Moat \Moat\, n. [OF. mote hill, dike, bank, F. motte clod, turf:
   cf. Sp. & Pg. mota bank or mound of earth, It. motta clod,
   LL. mota, motta, a hill on which a fort is built, an
   eminence, a dike, Prov. G. mott bog earth heaped up; or perh.
   F. motte, and OF. mote, are from a LL. p. p. of L. movere to
   move (see Move). The name of moat, properly meaning, bank
   or mound, was transferred to the ditch adjoining: cf. F. dike
   and ditch.] (Fort.)
   A deep trench around the rampart of a castle or other
   fortified place, sometimes filled with water; a ditch.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Moat \Moat\, v. t.
   To surround with a moat. --Dryden.
   [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):

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