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]

Moby Thesaurus II by Grady Ward, 1.0:

149 Moby Thesaurus words for "moat":
   abri, abysm, abyss, approach trench, aqueduct, arch dam, arroyo,
   backstop, bamboo curtain, bank, bar, barrage, barrier,
   bear-trap dam, beaver dam, boom, box canyon, breach, break,
   breakwater, breastwork, brick wall, buffer, bulkhead, bulwark,
   bunker, canal, canalization, canyon, cavity, channel, chap, chasm,
   check, chimney, chink, cleft, cleuch, clough, cofferdam, col,
   communication trench, coulee, couloir, countermine, coupure, crack,
   cranny, crevasse, crevice, crimp, cut, cwm, dam, defense, defile,
   dell, dike, ditch, donga, double sap, draw, dugout, earthwork,
   embankment, entrenchment, excavation, fault, fence, fire trench,
   fissure, flaw, flume, flying sap, fortified tunnel, fosse, foxhole,
   fracture, furrow, gallery, gap, gape, gash, gate, goffer, gorge,
   gravity dam, groin, groove, gulch, gulf, gully, gutter, ha-ha,
   hole, hydraulic-fill dam, incision, iron curtain, jam, jetty,
   joint, kennel, kloof, leak, leaping weir, levee, logjam, milldam,
   mine, mole, mound, notch, nullah, opening, parallel, parapet, pass,
   passage, pleat, portcullis, rampart, ravine, rent, rift, rime,
   roadblock, rock-fill dam, rupture, sap, scissure, seam, seawall,
   shutter dam, slit, slit trench, slot, split, stone wall,
   sunk fence, trench, trough, tunnel, valley, void, wadi, wall, weir,
   wicket dam, work