Area of a web page visible to the user's device they're using to view the page. For example, the viewport on a mobile device like a smartphone is a different aspect ratio than that of a desktop computer. On a desktop computer, the viewport is the rectangle in the browser window that displays a web document. In responsive web design, web content is designed to display differently according to the current size and orientation of the user's viewport.
The em and rem units are practical in creating perfectly scalable layout!
Viewport = the browser window size.
If the viewport is 50cm wide, 1vw = 0.5cm.