Mockup
A realistic visual representation of a design, used for presentation and evaluation before it is built.
In practice
A mockup is a realistic image of a design, often shown in context, used to present and judge it before it is built. It shows actual colors, type and imagery but usually does not work. Examples include a static picture of a new app screen inside a phone frame, or a label design placed on a photographed jar. Clients often find it easier to picture the final product from a mockup than from flat artwork.
Often confused with Prototype
A mockup shows how a design will look; a prototype lets people try how it works, through clicks, taps or physical use.
Try this
Place a flat label design onto a photo of a jar in an image editor, and compare it with the flat version.