A prototype is an early model of a product or deliverable. Creating a pre-production version allows evaluation, testing, and fixes before going to production or spending too much time on a faulty design.
Prototypes come in many varieties. They range lo-fidelity (lo-fi) to high-fidelity. An example of a lo-fi prototype is a paper-based version of what software will do -- a mock-up of the process and user interface. A high-fidelity is a custom-made version of a product that functions -- such as a car. In between the extremes are partially built models that show or test certain features. An example is a miniature scale architectural model that shows a building in relationship to the environment and shows the facade in detail. Another intermediate example is an engine that can be tested, rather than building the entire vehicle.
- Wikipedia on prototype