A viewport is the visible area of a web page that a user can see on their device's screen. It determines how content is displayed and can vary in size depending on the device, such as a computer, tablet, or smartphone. The viewport is essential for responsive design, allowing websites to adapt their layout to fit different screen sizes. In web development, the viewport is often controlled using the `meta` tag in HTML, specifically the `viewport` attribute. This helps developers set the width and scaling of the page, ensuring that users have an optimal viewing experience regardless of the device they are using.