Viewport

The visible window a person actually sees, whatever the full page size. You design for the frame, not the canvas.

The viewport is the actual window of attention: whatever the page's full size, the person sees one frame of it at a time, and everything outside that frame exists only as a promise. Designing for the viewport means composing what is visible together, not just what is adjacent in the file.

The frame, not the canvas reorders priorities: the opening viewport is the first impression entire, folds are real even if scrolling is easy, and elements that share a screen must be composed as sharing it. We check pages viewport by viewport at the real widths, because the canvas view in the design tool flatters arrangements no visitor will ever see at once.