When a search is performed in Google, the resulting SERP (Search Results Page) gives an estimate of the number of pages in Google’s index that match the query. For instance, in looking for “open source”, the resulting SERP is:
We can see the text “About 2,080,000,000 results” under the search box.
Besides, it is possible to restrict the search to a given site, by means of the “site:” prefix:
As we can see,google reports about 6,830,000 results for the query “open source” in en.wikipedia.org
Finally, it is possible to specify a query restricted to a given site, without a search string. This gives us an estimate of the total number of pages indexed for that site:
In this case, google reports about 16,400,000 pages indexed for en.wikipedia.org.
This last figure is what is commonly known as “indexation” of a web site.
In the previous example we saw that the first organic result for the “open source” query is “en.wikipedia.org”. In this case, we say that en.wikipedia.org, for that query, has a ranking of one.
That is, the ranking of a web site for a given query string on a given search engine is the position where it appears in the SERPs returned by the search engine for that query string.
Usually, a web site holds different types of content and we would want to know the ranking of the site for different search strings. Once the desired search strings have been defined and the ranking of the site for each of them has been obtained, the arithmetic average of those values can be computed. The average ranking is often known simply as “site ranking”.