The canonical tag was jointly introduced by Google, Yahoo and Microsoft earlier this year. Google hinted they would soon support cross domain canonical tag and they officially announced it last night. What is a canonical tag? It basically allows you to communicate to Google that a page has moved to a new location, without you using a server side 301 redirect.

Why is this important? A few reasons:

(1) Some hosts don’t allow webmasters to deploy 301 redirects
(2) Some site owners aren’t technical enough to implement a 301 redirect
(3) In some cases, webmasters do not want to redirect users but rather only search engines (i.e. pagination, weird filtering, tracking parameters added to URLs, etc).

Initially this was launched to work within a domain only. So you could only say that domainA.com/pageA has moved to domainA.com/pageB. Now you are able to do this across domains, so you can say, domainA.com/pageA has moved to domainB.com/pageA or even domainA.com moved to domainB.com.

Google made this cute graphic to explain it:

There is a lot of technical detail on how to implement this tag over here but in short, you just add this tag to the head section of your HTML of the page you want to redirect. Make sure the URL you specify is the URL you want GoogleBot to be redirected to:

JohnMu, who wrote the Google blog post also put some FAQs, and note the “hint” he keeps referring to. I assume the “hint” will become less of a hint and more of an “instruction” as time goes on.

Forum discussion at Google Webmaster Help.