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.

What a Prototype Is (and Is Not)
UX Magazine
The Right Tool For The Job: Picking The Best Prototyping Software For Your Project
How to Choose a UX Prototyping Tool
Design For Use
Considering Prototypes
UX Booth

Design Better And Faster With Rapid Prototyping
Smashing Magazine
Wireframes are dead, long live rapid prototyping
UX for the masses
Are Rapid Prototyping, Lean UX, and Agile Development Good for User Experience?
UX Matters
6 Ways to Save Time in Rapid Prototyping
Invision Blog