A wall is a structure and a surface that defines an area; carries a load; provides security, shelter, or soundproofing; or serves a decorative purpose. There are various types of walls, including border barriers between countries, brick walls, defensive walls in fortifications, and retaining walls that hold back dirt, stone, water, or noise. Walls can also be found in buildings, where they support roofs, floors, and ceilings, enclose spaces, and provide shelter and security.

The construction of walls can be categorized into framed walls and mass-walls. Framed walls transfer the load to the foundation through posts, columns, or studs and typically consist of structural elements, insulation, and finish elements. Mass-walls are made of solid materials such as masonry, concrete, adobe, or rammed earth. Walls may also house utilities like electrical wiring or plumbing and must conform to local building and fire codes.

Walls have historically served defensive purposes, and the term "wall" originally referred to defensive walls and ramparts. Examples of famous defensive walls include the Great Wall of China and Hadrian's Wall. Walls can also be used for decorative or aesthetic purposes.

The term wall comes from the Latin vallum meaning "an earthen wall or rampart set with palisades, a row or line of stakes, a wall, a rampart, fortification", while the Latin word murus refers to a defensive stone wall. English uses the same word to mean an external wall and the internal sides of a room, but this is not universal, and many languages distinguish between the two. This distinction can be seen in German between wand and mauer, and in Spanish between pared and muro.

The word wall originally referred to defensive walls and ramparts.

The purposes of walls in buildings are to support roofs, floors and ceilings; to enclose a space as part of the building envelope along with a roof to give buildings form; and to provide shelter and security. In addition, the wall may house various types of utilities such as electrical wiring or plumbing. Walls may or may not be load-bearing. Walls are required to conform to the local building and/or fire codes.

Wall construction falls into two basic categories: framed walls or mass-walls. In framed walls, the load is transferred to the foundation through posts, columns or studs. Framed walls most often have three or more separate components: the structural elements (such as 2×4 studs in a house wall), insulation, and finish elements or surfaces (such as drywall or panelling). Mass-walls are of a solid material, such as masonry, concrete including slipform stonemasonry, log building, cordwood construction, adobe, rammed earth, cob, earthbag construction, bottles, tin cans, straw-bale construction, or ice.

There are three basic methods through which walls control water intrusion: moisture storage, drained cladding, or face-sealed cladding. Moisture storage is typical of stone and brick mass-wall buildings where moisture is absorbed and released by the walls of the structure itself. Drained cladding, also known as screened walls, acknowledges moisture will penetrate the cladding so a moisture barrier such as housewrap or felt paper inside the cladding provides a second line of defense, and sometimes a drainage plane or air gap allows a path for the moisture to drain down through and exit the wall. Sometimes ventilation is provided in addition to the drainage plane such as in rainscreen construction. Face-sealed cladding, also called barrier wall or perfect barrier cladding, relies on maintaining a leak-free surface of the cladding. Examples of face sealed cladding are the early exterior insulation finishing systems, structural glazing, metal clad panels, and corrugated metal.