Official Google Webmaster Central Blog: Raising awareness of cross-domain URL selections
Webmaster level: Advanced
A piece of content can often be reached via several URLs, not all of which may be on the same domain. A common example we’ve talked about over the years is having the same content available on more than one URL, an issue known as duplicate content. When we discover a group of pages with duplicate content, Google uses algorithms to select one representative URL for that content. A group of pages may contain URLs from the same site or from different sites. When the representative URL is selected from a group with different sites the selection is called a cross-domain URL selection. To take a simple example, if the group of URLs contains one URL from a.com and one URL from b.com and our algorithms select the URL from b.com, the a.com URL may no longer be shown in our search results and may see a drop in search-referred traffic.