Glossary/Drawing basics
Viewport
A window on a paper space layout that shows a scaled view into model space.
A (paper space) viewport is a rectangular "window" placed on a layout that displays model space geometry at a chosen scale — this is how the same drawing can appear at 1:50 on one sheet and 1:20 on a detail sheet without redrawing anything.
A layout can have more than one viewport (e.g. an overall plan plus a detail callout), and what's visible through each can be controlled independently with layer and viewport-specific overrides.