The visual design of a screen as it will look — real colours, type, images and content — the stage after wireframes and before development.
It is the cheapest place to disagree. Changing a mockup takes an hour; changing the same thing after development takes a sprint and a renegotiation.
We review mockups with real content and the awkward cases — longest name, empty list, error state — because a design approved only on ideal content is approved on a situation that will not occur.