A joist, in architecture

Architecture

Architecture is both the process and product of planning, designing and construction. Architectural works, in the material form of buildings, are often perceived as cultural and political symbols and as works of art...

 and engineering

Engineering

Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...

, is one of the horizontal supporting members that run from wall to wall, wall to beam, or beam to beam to support a ceiling

Ceiling

A ceiling is an overhead interior surface that covers the upper limit of a room. It is generally not a structural element, but a finished surface concealing the underside of the floor or roof structure above....

, roof

Roof

A roof is the covering on the uppermost part of a building. A roof protects the building and its contents from the effects of weather. Structures that require roofs range from a letter box to a cathedral or stadium, dwellings being the most numerous....

, or floor

Floor

A floor is the walking surface of a room or vehicle. Floors vary from simple dirt in a cave to many-layered surfaces using modern technology...

. It may be made of wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

, steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

, or concrete

Concrete

Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

. Typically, a beam

Beam (structure)

A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...

 is bigger than, and is thus distinguished from, a joist. Joists are often supported by beams and are usually repetitive.

The wider the span between the supporting structures, the deeper the joist will need to be if it is not to deflect under load.