It's a slang term, it has no official recognition. Google have never described a site an an authority site.
Every few years old site with good content and strong backlinks is supposed to be an "authority site" in Google. For me an authority site is a site known by many in some particular niche. ProBlogger, Smashing Magazine, etc.
There is no such an "Authority Site" once you have a good site with quality content and a good inner linking structure and ranking #1 for any keyword like in most cases your domain name or even your brand... Then your site will be listed on the SERP's with some extra links to master inner pages in your website right under the regular listing. To make it clear it's called ( Site Links ) and you can read more about it here: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=47334 and here: http://googlewebmastercentral.blogspot.com/2006/09/information-about-sitelinks.html in some cases if you are lucky then you will find a site search box as well
When I think of authority sites, I think of the most well-linked and content rich pages in my niche. Authority sites generally are very rich in content, aged several years, and do very well in the SERPs.
Authority site is just a term for a site that has huge link popularity, because lots and lots and lots of websites are pointing to it. If 20,000 different sites are pointing to a domain, that entire domain will have a lot more weight. Google also gives more weight to sites like .gov. A link from a PR 1 or 2 deep page on an authority site will be far more valuable than a PR 1-2 page from a regular old site.
website which are having site links, and one of the brand for the particular keyword. like: "shop" is the keyword and search in Google with it and see the first result containing site links, is an authority website.
According to who? that might be what YOU call an authority site, but it has no official sanction at all. Particularly as google are using sitelinks more and more - they use them because they think they make it easier for searchers to find what they want - it's got nothing at all to so with some imaginary award you think google is giving to sites.
There is no official statement from Google about site authority status. However, I think sitelinks has something to do with authoritative status. Why? I see there is only a few sites which Google show their inner page as sitelinks, and most of them are ranked #1 in many competitive search term. Did you ever see the site which ranked #2 or below show sitelinks? I never seen it before. Not all site ranked #1 show sitelinks but some with quality link to inner page will.
I think you can pretty much assume PR 8 and above are authority sites. These sites Google trusts and indexes new pages instantly
IMO, what i feel i wrote, Still i really dis like your comment on it, I think you need to take a short look at here only... http://www.graphicwebdesign.net/content/view/66/56/
Links brokers call it "authority website"; Google doesn't distinguish between websites in that way. it depends on the age, relevant backlinks, content, branding, traffic...... Usually EDUs websites are called authority websites (NOT all of them for sure) , coz simply an edu website might be listed in good related academic website or magazine, and not in a Viagra shop. Feras,