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.

How To and Tools:
Wireframes are dead, long live rapid prototypingUX for the Masses
The Right Tool For The Job: Picking The Best Prototyping Software For Your ProjectUX CollectiveJuly 2nd, 2016
6 ways to save time in rapid prototypingInvision BlogMarch 30th, 2015
What a Prototype Is (and Is Not)UX MagazineNovember 17th, 2014
Are Rapid Prototyping, Lean UX, and Agile Development Good for User Experience?UX MattersMarch 18th, 2013
How to choose a UX prototyping toolDesign for UseSeptember 16th, 2011
Design Better And Faster With Rapid PrototypingSmashing MagazineJune 16th, 2010
Considering PrototypesUX BoothJune 15th, 2010