A wall is a usually solid structure that defines and sometimes protects an area. Most commonly, a wall delineates a building and supports its superstructure, separates space in buildings into rooms, or protects or delineates a space in the open air. There are three principal types of structural walls: building walls, exterior boundary walls, and retaining walls.
Building walls frequently become works of art externally and internally, such as when featuring mosaic work or when murals are painted on them; or as design foci when they exhibit textures or painted finishes for effect.
On a ship, the walls separating compartments are termed "bulkheads", whilst the thinner walls separating cabins are termed "partitions".
Boundary walls include privacy walls, boundary-marking walls on property, and city walls. These intergrade into fences; the conventional differentiation is that a fence is of minimal thickness and often is open in nature, while a wall is usually more than a nominal thickness and is completely closed, or opaque. More to the point, if an exterior structure is made of wood or wire, it is generally referred to as a fence, while if it is made of masonry, it is considered a wall. A common term for both is barrier, convenient if it is partly a wall and partly a fence, for example the Berlin Wall. Another kind of wall/fence ambiguity is the ha-ha which is set below ground level, so as not to interrupt a view yet acting as a barrier to cattle for example.
Before the invention of artillery, many European cities had protective walls. In fact, the English word "wall" is derived from Latin vallum, which was a type of fortification wall. Since they are no longer relevant for defense, the cities have grown beyond their walls, and many of the walls have been torn down. Extreme examples of boundary walls include the Great Wall of China and Hadrian's Wall. A modern functional example was the Berlin Wall, which divided Germany.
In areas of rocky soils around the world, farmers have often pulled large quantities of stone out of their fields to make farming easier, and have stacked those stones to make walls that either mark the field boundary, or the property boundary, or both.
Retaining walls are a special type of wall, that may be either external to a building or part of a building, that serves to provide a barrier to the movement of earth, stone or water. The ground surface or water on one side of a retaining wall will be noticeably higher than on the other side. A dike is one type of retaining wall, as is a levee, a load-bearing foundation wall, and a sea wall.