Prototype
An interactive or tangible model of a design used to test its functionality, flow, and user experience.
In practice
A prototype is a working model of a design that people can actually use, built at whatever fidelity answers the question at hand. It can be a few paper screens a tester taps, a clickable app flow, or a foam handle shaped to test grip. Prototypes reveal problems that pictures hide: a checkout that looked clear in a mockup may confuse people the moment they try it.
Often confused with Mockup
A prototype can be used and tested for function and flow; a mockup only shows what the design will look like.
Try this
Draw three phone screens on paper, have a friend tap through them, and swap the sheets as they go.