A prototype is an original type, form, or instance of some thing serving as a typical example, basis, epitome, or standard for other things of the same category.

'In semantics, prototypes or prototypical instances combine the most representative attributes of a category. They are the best examples among the members of a category and serve as benchmarks against which the surrounding "poorer" instances are categorized (see Prototype Theory).

Design and modeling
In many fields, there is great uncertainty as to whether a new design will actually do what is desired. New designs often have unexpected problems. A prototype is built to test the function of the new design before starting production of a product.

Building the full design is often expensive and can be time-consuming. Sometimes, rather than building the full design, figuring out what the problems are, then building another full design, "rapid-prototyping" or "rapid application development" techniques are used for the initial prototypes, which implement part, but not all, of the complete design. This allows manufacturers to rapidly and inexpensively test the parts of the design that are most likely to have problems, solve those problems, and then build the full design.

This counter-intuitive idea -- that the quickest way to build something is, first to build something else -- is shared by scaffolding and the telescope rule.

Automobile and automobile racing
Automobiles from the premier tier of international sports car racing are informally called prototypes. Unlike GT automobiles, which are based on road-legal, production models that are modified for racing, these prototypes are designed purely for racing and track use?in order to expand the limits of high performance. The enormous expense of building and racing these automobiles is rewarded in status achieved by the builder, usually resulting in sales of expensive production automobiles implementing many of the elements developed for racing in prototypes?rather than production of the models raced?or, through name recognition alone (see Audi R8).

The overall winners of multi-class events such as the 24 Hours of Le Mans usually are?or were?prototypes, as were the Ferrari P series, the Porsche 908, or the Alfa Romeo 33. In contrast, the famous Porsche 917 and Ferrari 512S, featured in the 1971 movie, Le Mans, were entered in another class?the "sports car" class?a class for which a minimum of twenty-five of each automobile model had to be built to qualify ("homologation").

A contemporary example of an automobile racing prototype is the Audi R10 Diesel.

Other mechanical and electrical engineering
The most common use of the word prototype is a functional, although experimental, version of a non-military machine (e.g., automobiles, domestic appliances, consumer electronics) whose designers would like to have built by mass production means, as opposed to a mockup, which is an inert representation of a machine's appearance, often made of some non-durable substance.

In the fictional Gundam universe, prototypes have many superb features which are not to be included in their mass production models because their high performance is proportional to their high cost. This somewhat unlikely setting is a compromise between realism and heroic story lines: the only leading characters can pilot powerful mobile suits.

Builders of military machines and aviation prefer the terms "experimental" and "service test".

Computer science
In computer science, a function prototype is the declaration of a subroutine or function. However, in prototype-based programming (in the context of object-oriented programming), a prototype is an object that can be cloned in order to produce new objects.

It can also refer to the Prototype JavaScript framework.

Computer software engineering
In Software Engineering, a prototype generally refers to a breadboard (or evolutionary) prototype or a throwaway (or one-off) prototype. Breadboard prototypes are often software in a development stage, focusing on a subset of the total requirements for a product. These prototypes usually are intended to evolve into the final design. Project managers may formally identify a software component as prototype to communicate with stakeholders that the component may or may not comprise the techniques ultimately allocated to the product design, or to meet business objectives. It should not be assumed that the prototype is merely for testing concepts (throwaway). That would be an aspect of a "research" project or "proof of concept." Prototypes provide the software developers with a "working model" for demonstration or use by customers, quality-assurance, business analysts, and managers to confirm or make changes to requirements, help define interfaces, develop collaborating components, and to provide proof of incremental achievement of scheduled contractual agreements. Software Prototyping serves any and all of these purposes in practice.

Extreme Programming uses iterative design to gradually add one feature at a time to the initial prototype, attempting to minimize "irreducible complexity".

Model railroads
Among model railroad enthusiasts, at least in North America, actual railroads are commonly spoken of as "prototypes".

Scale modeling in general
In the field of scale modeling (which includes model railroading, vehicle modeling, airplane modeling, military modeling, etc.), a prototype is the real-world basis or source for a scale model?such as the real EMD GP38-2 locomotive?which is the prototype of Athearn's (among other manufacturers) locomotive model. Technically, any non-living object can serve as a prototype for a model, including structures, equipment, and appliances, and so on, but generally prototypes have come to mean full-size real-world vehicles including automobiles (the prototype 1957 Chevy has spawned many models), military equipment (such as M4 Shermans, a favorite among US Military modelers), railroad equipment, motor trucks, motorcycles, airplanes, and space-ships (real-world such as Apollo/Saturn Vs, or the ISS).

There is debate whether 'fictional' or imaginary items can be considered prototypes (such as Star Wars or Star Trek starships, since the feature ships themselves are models or CGI-artifacts); however, humans and other living items are never called prototypes, even when they are the basis for models and dolls (especially - action figures).

In the science and practice of metrology, a prototype is a human-made object that is used as the standard of measurement of some physical quantity to base all measurement of that physical quantity against. Sometimes this standard object is called an artifact. In the International System of Units (SI), the only prototype remaining in current use is the International Prototype Kilogram, a solid platinum-iridium cylinder kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) in Paris that, by definition is the mass of exactly one kilogram. Copies of this prototype are fashioned and issued to many nations to represent the national standard of the kilogram and are periodically compared to the Paris prototype.

Until 1960, the meter was defined by a platinum-iridium prototype bar with two scratch marks on it (that were, by definition, spaced apart by one meter), the International Prototype Metre, and in 1983 the meter was redefined to be the distance covered by the speed of light in 1/299792458 of a second (thus defining the speed of light to be 299792458 meters per second).

It is widely believed that the kilogram prototype standard will be replaced by a definition of the kilogram that will define another physical constant (likely either Planck's constant or the elementary charge) to a defined constant, thus obviating the need for the prototype and removing the possibility of the prototype (and thus the standard and definition of the kilogram) changing very slightly over the years because of loss or gain of atoms.