A mirror is a Web site or set of files on a computer server that has been copied to another computer server so that the site or files are available from more than one place. For files that are popular for downloading, a mirror helps reduce network traffic, ensures better availability of the Web site or files, or enables the site or downloaded files to arrive more quickly for users close to the mirror site. Mirroring is the practice of creating and maintaining mirror sites.
A mirror site is an exact replica of the original site and is usually updated frequently to ensure that it reflects the content of the original site. Mirror sites are used to make access faster when the original site may be geographically distant (for example, a much-used Web site in Germany may arrange to have a mirror site in the United States). In some cases, the original site (for example, on a small university server) may not have a high-speed connection to the Internet and may arrange for a mirror site at a larger site with higher-speed connection and perhaps closer proximity to a large audience.
In addition to mirroring Web sites, you can also mirror files that can be downloaded from an File Transfer Protocol server. Netscape, Microsoft, Sun Microsystems, and other companies have mirror sites from which you can download their browser software.
Mirroring could be considered a static form of content delivery.
Also see port mirroring.