Mirror term explained
A full copy of the site on another (not main) domain. A copy of the site occurs as a result of the transfer of the site to a new domain or for reasons of load distribution on servers. The site copy is called a mirror of the main site.
Mirrors can be a lot, but in the search results when a successful joining of the domain is shown only the main thing the main mirror.
For the bonding of mirrors in the search engines there is a special mirror-robot that comes to the site in its special algorithm. To anticipate the arrival of mirror-robot quite difficult, so sometimes to make changes in the search engines index you have to wait a considerable time.
Do not confuse the concept of a mirror site and aliases of sites. Despite the fact that the words are almost synonyms, they have different purposes. Mirrors are created mainly to distribute the load between servers or traffic between the regions of the users, while aliases are synonymous with domain names (e.g. www.seoheronews.com and seoheronews.com).