The word prototype is derived from the Greek word "prōtotupos", which meant "a first or primitive form." In Medieval Latin, the word evolved to mean "original, primitive", and today, it is referred to mostly as a key artifact in the design process of any product, be it physical or digital.

The fifteenth-century Italian architect, Leon Battista Alberti describes in his book On the Art of Building in Ten Books, an event from the first-century BC in which Julius Caesar,

Julius Cesar, the ruler of the Roman Empire, was probably not acting in the capricious, short-tempered manner of an autocrat. Rather, we can think of him as a typical client, reacting negatively to a design that did has not been reviewed with him prior to building, and the end result did not meet his expectations.

Prototype help solve a serious communication challenge--How can the designer share ideas about the look and feel of the product with clients and other stakeholders so that the latter can evaluate the fit of the approach to the product's needs, afford feasibility assessments, and approve or modify prior to moving ahead to the detailed design.

Alberti expresses this through this quote:

Despite the reference to buildings, this text sounds very contemporary and relevant to experience prototyping in general. Alberti outlines the benefits of prototyping, such as the ability to articulate the layout, the hierarchy of entities, the organization of elements, the order of entities, and also the ability to use the prototype for the estimation of complexity and cost.

The preceding screenshot is of Axure RP, a popular prototyping tool used by UX designers who create user experiences for software, be it for web or mobile applications. The software enables designers at all skill levels to create prototypes with various degrees of fidelity--from simple click-troughs, which are similar to a PowerPoint slide show, to highly detailed interactive simulations of the intended software.

Prototyping software, in general, should support team collaboration for multi-designer projects, simulate desktop and mobile experiences, and support Macintosh, Windows, and all common browsers.

Some key experiences that designers can communicate with prototyping software products are as follows:

• Action and response: The prototype needs to simulate key scenarios that a user would perform, and the product's appropriate responses to actions the user is taking. Scenarios could involve multiple steps and include branching subscenarios based on contextual conditions. The prototype should demonstrate these interaction and the user's ability to complete them in a satisfactory way.
• Context awareness: This is a personalized experience based on login; the prototype needs to simulate how the system will render for different users, based on the user role and entitlements. Common examples are e-commerce and banking applications. For non-registered users, such sites provide a "public" face and an experience that is somewhat generic. They may display special offers to entice users to register. A registered user may get information based on previous purchases, preferences they have set in an earlier session, and so on. A paying subscriber will have access to additional content, and recommendations based on their past activity.
• Scalability and future scope: Applications are developed and deployed in phases, allowing the company to prioritize its investment in the product, based on strategic goals, practical budgetary constraints, and technical constraints. The prototype is often conceived to demonstrate how future capabilities and functionality will be incorporated in the framework.
• Localization support: To reach a global audience, many applications must support the language and sometimes the cultural preferences of the locale of its users are from. The prototype can demonstrate how various elements, such as navigation or buttons, handle languages that are written from left to right and right to left, and accommodate languages that have longer words than English, such as German or Spanish.
• Error handling: How does the system communicate to the user various types of user or system errors, and where and how do errors and alerts appear on the screen?

The image above includes several examples of prototyping software used for the design of physical products.

This type of software--CAD (computer aided design), has been around for several decades; originally it was just used by aircraft designers and architects, but is now a common design instrument.

The preceding image shows a three-dimensional method of prototyping in which clay is used to reproduce the product's external features. This technique is common in the automobile industry. Working with clay is similar to sculpture, except that the material is a lot more forgiving than marble.

Finally, a few examples of the most recent entry to the prototyping tools, such as 3D printing, are shown in the preceding image. As the price of this technology decreases and with the level of sophistication, detail, and complexity the devices can produce increases, designers are able to experiment with much more risk-taking than before.