Indicates a resource that is designed to be a mirror or surrogate of the object resource; a backup copy, should the object resource be lost or unobtainable.
Consider a resource at http://flickr/set/image1.jpg - if you make a backup or mirror of this resource at a resolvable location, such as http://mirror/flickr_bkup1.jpg, you can indicate it like this:
<http://mirror/flickr_bkup1.jpg> <o:mirror> <http://flickr/set/image1.jpg>