A mockup is a static, visually polished design of a screen – with final colours, typography and graphics but no working interactions. It shows what the product will look like before it is built.
In the fidelity hierarchy: sketch → wireframe (structure) → mockup (appearance) → prototype (behaviour). A mockup is a "photograph" of the future product: the client approves the visual direction from it, and developers estimate the build.
Good practice: design mock-ups with real content (not lorem ipsum) and real conditions – long names, missing photos, a phone screen. A beautiful mockup with perfect data is a promise production won't keep.
The boundaries blur nowadays: tools like Figma turn mock-ups into clickable prototypes with one gesture.