Mirror Site: A mirror site is a website that contains identical content to another website. This content is usually "mirrored" in order to serve as a duplicate or backup of the original site's content.
While it is true that search engines like Google try not to index duplicate content on the web (or penalize it when it's found), mirror sites can sometimes pose an exception to this rule. A mirror site is useful in case one website goes down, often due to a sharp increase in visitors.
Mirror sites can act as a way to provide multiple sources of the same information to users. Some mirror sites are based in different countries, so a user who can't access a U.S. site due to too much traffic can use a site for Canada instead.
Mirror sites are especially popular for websites that host downloads. A good example of this is Project Gutenberg, which is an open project that hosts downloads for public domain literary works. The site has several mirrors for HTML, plain text, and mobile device books.