In common usage, a mockup is a scale model of a structure or device, usually used for teaching, demonstration, testing a design, etc.
A mockup should not be confused with a prototype. Prototypes are always meant to function, even if not fully so, whereas mock-ups are only meant to look like the real system, and do not function. The two are easy to confuse in software and systems engineering especially, where mockups are a way of prototyping user interfaces on paper or computer images. In many cases it is best to design the user interface before code is written or hardware is built, to avoid having to go back and make expensive changes.
Mockup is also a frequently used term when talking about an early layout or sketch of a Web site or GUI program.
Mockups are commonly required by designers, architects, and end users for custom furniture and cabinetry. The intention is often to produce a full sized replica, using inexpensive materials in order to verify any number of things. Often done to determine the proportions of the piece, relating to various dimensions of the piece itself, or to fit the piece into a specific space or room. The ability to verify the how the design of the piece relates to the rest of the space is also an important factor in determining size and design.
When designing a functional piece of furniture, such as a desk or table, testing clearances for function regarding the standard sizes of the human form is another aspect of the value of mockups. Designs that fail to consider these standards are more likely to be a failure regarding practical use.
Another purpose of mockups is the ability to verify color, finish, and design details. These details cannot be determined by the drawings and sketches as typically done as the piece is being transformed from an idea to a work in progress. These mockups often can be done on a miniature version of the full sized piece.
In general, a mock up adds to the initial cost of the project, but results in an overall savings and avoids misinterpreted concepts.