[Is it those which have high PR + 100+ indexed pages + on google top 10? I ask this for keyword research. If I type a potential keyword in google I would like to, using SEO Quake, be able to recognize which are the authority sites in that top 10.]
This is one of those questions that you are likely to get a different answer from each member replying. So please allow me to give my answer, which will probably differ from most others.
We can all come up with our own definition for what we personally regard as an authority. However, how useful it to define it any other way than the way search engines define it?
Google measures authority with its PageRank algorithm, however, if you think you can just look at the PR of a page and know how much authority it holds for a particular keyword you are wrong. Typically, that collective authority is spread out over a number of keywords and it might hold very little authority for the particular keyword you are analyzing. You must dig a little deeper to get meaningful data.
I believe authority should be viewed as a relative value. For example, a page with little PR could be the most authoritative page in the index, for a particular keyword, if there are none with greater relevant backlinks. Relevancy is paramount in determining page authority for a particular keyword. Irrelevant backlinks may add PR to a page but not improve that page’s authority for a particular keyword.
The quickest and easiest way to see how a page compares, relative to others, for authority, is to use the allinanchor: search operator with the keyword. Google search will return results ranked in order of authority for the keyword you are analyzing.