Moat

Moat, a ditch filled with water surrounding a castle, town or other fortified place for purposes of defence. The word is taken from the O. Fr. mote, or motte, a mound or embankment of earth used as a means of defence; the transition in meaning from the heap of earth to the trench left by excavating the earth is parallel with the similar interchange of meaning in dike and ditch (see DIKE). In mod. Fr. motte means a lump or clod of earth. The word is probably of Teutonic origin, and may be connected with Eng. "mud." (See FORTIFICATION AND SIEGECRAFT.)