Short answer
A prototype is a clickable simulation of a product that behaves enough like the real thing to be tested with users, while containing no working back end. Buttons navigate and forms accept input, but the data is fixed. The purpose is to find confusion in a flow while changing it still costs an afternoon.
Also called: Clickable prototype, Interactive prototype
Fidelity is a dial rather than a setting. Paper sketches surface gross misunderstandings for the cost of an hour; a clickable wireframe tests whether a sequence makes sense; a high-fidelity prototype with real copy and interaction tests whether someone can finish a task unaided. Each step upward buys better feedback and costs more, and past a certain point reviewers start discussing features instead of the flow.
Two hazards are worth naming. The first is testing with the people who built the thing, which reliably confirms that the flow is obvious to those it is already obvious to; a handful of outsiders attempting a real task is worth more than any internal walkthrough. The second is promoting a prototype to production. Prototype code is written to be discarded, and the pressure to ship it is strongest exactly when the demonstration has gone well.
Common questions
Is a prototype the same as an MVP?
No. A prototype simulates the product with fixed data and no working back end, and exists to be tested and thrown away. An MVP is live software handling real data for real users, however narrow its scope. Confusing the two is how a demo built in a fortnight ends up carrying customer records.
Where this comes up in our work
Related terms
Wireframe
A wireframe is a low-fidelity layout of a screen showing structure, hierarchy and content placement without colour, typography or imagery.
Minimum Viable Product (MVP)
A minimum viable product is the smallest version of a product that delivers real value to a real user and returns evidence about whether the idea works.
User Acceptance Testing (UAT)
User acceptance testing is the stage where the people who will actually use a system verify it against the agreed requirements before launch, working through real scenarios on a staging environment.
Reading definitions because you are scoping a project? Skip ahead and just ask.