a.k.a. mirror site -or- mirror server -or- white label
This is either a domain name that another domain points to or a server that holds copies of a different domain's files. Some servers are so popular that other servers have been set up to mirror them, in order to spread the traffic load to more than one site. Many international sites have mirrors set up in other countries to allow quicker access for international users.
An archive site or Web site that keeps a copy of some or all files at another site-to make them more quickly available to local users and to reduce the load on the source site-is also an example of mirroring. Such mirroring is usually done for specific directories or files on a specific remote server (as opposed to the way cache or proxy servers keep copies of everything that is requested).