¿Qué es un prototipo?

El artículo What a Prototype Is (and Is Not) explica muy bien cómo usar un prototipo.

Un prototipo no es:

A prototype is not the final product. Do not expect it to look like the final product. It need not have a high fidelity or be pixel perfect.

Un prototipo sí es:

It’s a simulation of the final product. It’s like an interactive mockup that can have any degree of fidelity. The main purpose of building prototypes is to test whether or not the flow of the product is smooth and consistent.