A working demo of the envisioned interaction/behaviour. A prototype can range from flow to micro-animation. The fidelity of the prototype is not fixed, it can range from a couple of wireframes attached with hotspots to a fully coded app. Guideline is that prototypes generally take less time to make than the final product, but have enough detail to judge the interaction.
We need a prototype of this interaction before we can make a decision
To make a prototype
Let's prototype this flow so we can show it to users
See also: Mockup