In CSS, the term "viewport" refers to the visible area of the browser where your website is displayed. Understanding the viewport in CSS is important for creating responsive designs that look good on various devices and screens.

Here are some key things to know about viewport in CSS:

1. Viewport width and height: You can set the width and height of the viewport using CSS properties like width and height. For example:

   width: 100%;
   height: 100vh; /* 100% viewport height */
   

2. Viewport meta tag: In HTML, you can set a viewport meta tag that defines how the browser should scale and display the content on mobile devices. For example:

   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   

3. Media queries: You can use media queries in CSS to adjust styles based on the size of the viewport. For example:

   @media (max-width: 768px) {
       /* Styles for screens up to 768px wide */
   }
   

4. Viewport units: CSS provides units that are relative to the size of the viewport, such as vw (1% of viewport width), vh (1% of viewport height), vmin (the smaller of vw or vh), and vmax (the larger of vw or vh).

5. Scrolling: Viewport dimensions can be important when styling scrolling behavior, such as when using overflow or positioning elements fixed within the viewport.