A thumbnail is a reduced preview of an image or video that is used on websites or in software applications. Thumbnails are often used in galleries, lists or search results to give users a quick overview of the content before they open the image or video in full size.

Thumbnails are usually displayed as small square or rectangular images and are generally only a few kilobytes in size to ensure fast loading speed. Most thumbnails show the original image in a reduced form or in a cropped version to show the essence of the content.

Thumbnails are a useful feature in web development and can help improve the user experience by giving users a quick way to preview content without having to open it full size. Thumbnails can also help to improve the loading speed of the website as they take up less memory and less time to load than the full image or video.

As a rule, thumbnails are generated automatically when an image or video is uploaded to a website. In some cases, however, customised thumbnails can also be created to highlight certain elements of the content or to ensure a better display in the preview.

Overall, thumbnails are an important function in web development that can help to improve the user experience and optimise the loading speed of websites.