Prototype
An early representation of a product, service or system used to explore, communicate and test assumptions before full implementation.
Prototypes let teams test whether users understand and want a design before engineering budget is committed, which is far cheaper than reworking a released product or feature after launch.
A SaaS team builds a clickable Figma prototype of a new onboarding flow, runs moderated tests with 5 target users, and finds that 3 of them miss the integration step, so the flow is redesigned before any code is written.
A prototype only answers the questions it was built to test. High-fidelity prototypes can draw feedback on visual polish instead of the concept, and a successful test does not prove technical feasibility, performance or real-world adoption.
Prototypes vary by fidelity (paper sketches, wireframes, clickable mockups, coded prototypes) and by scope, from a horizontal prototype that covers many screens shallowly to a vertical prototype that implements one path in depth. Each round should be tied to a hypothesis and a test method such as usability testing or a Wizard of Oz study, and the findings feed the next iteration.
Systems Architecture
Nielsen Norman Group — UX Prototypes: Low Fidelity vs. High Fidelity — https://www.nngroup.com/articles/ux-prototype-hi-lo-fidelity/; Interaction Design Foundation — What is a Prototype? — https://ixdf.org/literature/topics/prototypes