A mirror site is a near identical duplicate website or webpage. Mirrors are commonly used in an effort to target different keywords and keyphrases. Using mirrors is a violation of the Terms Of Service of most search engines and could be grounds for banning.

Hi,
Mirror site is an exact copy of another Internet site.

In simple way, a mirror is a Web site on a computer server that has been copied to another computer server so that the site are available from more than one place.

HI
mirror sites are the duplicate sites which are made same as some high PR sites, but this is a balck hat technique.