Wireframe.

A wireframe is a basic blueprint of a screen: boxes, labels, and placement, without color, imagery, or polish. It answers what goes where and how a flow works, before anyone worries about how it looks.

Wireframes are cheap and fast to change, which is exactly the point. It's far easier to rearrange a rough sketch than to redo a finished design or, worse, rebuilt code. They align everyone on structure early.

Once the wireframes are right, designers add the visual layer and engineers build from a plan that's already been thought through. Skipping this step is how teams end up redesigning live screens.