A deep, wide ditch surrounding a castle, fort, town, etc., typically filled with water and intended as a defence against attack. Also in extended use.

A moat or arm of the sea. Obsolete. rare.

A ditch running through or around a town; esp. a ditch or moat surrounding a walled town.

spec. A ditch constructed as a defensive barrier in front of or around a building, settlement, etc.; a moat encircling a fortified place.

A fosse (in various senses); esp. a defensive ditch.

A protective barrier or defence consisting of a tract of water. rare.