A mirror site is a website which is the same as another website operated by the same person ororganization but has a slightly different address. Mirror sites are designed to make it easier for more people to visit a popular website.
mirror site in British English
(ˈmɪrə saɪt)
noun
a website that is the same as another website operated by the same person or organization, designed to make it easier for more people to visit a popular website