“Let’s start using the terms ‘prototype’ and ‘mockup’ to mean different things. In my opinion, they talk about different ends of the problem.
A mockup shows us a possible solution. It asks the question, “What do you think of this direction for our design?” Whether the mockup is a sketch or a fully-rendered interactive experience, it’s looking at whether the ideas contained would be a good solution.
Prototypes are different from mockups. They don’t focus on the solution, but on understanding the problem. They ask the question, “What happens when we try this?” Maybe we learn it’s the right idea, but more likely we learn something about the problem we didn’t know before.”