What is a viewport in the context of the web?

Viewport is the visible area of a web page within the device's window (browser, phone, tablet). What the user actually sees without scrolling is the viewport.

Features:

• on desktop, the viewport ≈ the browser window;
• on mobile, the viewport is usually smaller than the site's full width, and the browser may scale the page;
• the viewport size directly affects responsive layout, media queries, and element behavior.

The viewport is most often controlled via a meta tag:

It tells the browser:

• use the actual device width (width=device-width),
• not scale the page by default (initial-scale=1.0).

Summary: viewport is the "screen window" inside which the page is displayed, and it is exactly its dimensions that are used for responsive design.