Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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