Full duplicate website, located on a different, different from the main, domain name.

Most often, mirrors are created to reduce the load on the main web resource.

A rarer reason for creating mirrors is the backup storage of data in case of malfunctions on the server hosting the main website.

Regardless of the number of site mirrors, they all go through the stage bonding with the address of the main resource, which as a result is displayed in searchable issues.

Sometimes there is an incorrect definition by search engines of the main mirror. To avoid this, you need to in the file robots.txt all mirrors give an indication with the desired address for search engine robots.