Google has no single authority metric but rather uses a bucket of signals to determine authority on a page-by-page basis.
Google’s fight against problematic content has drawn renewed attention to a common question: how does Google know what’s authoritative? The simple answer is that it has no single “authority” metric. Rather, Google looks at a variety of undisclosed metrics which may even vary from query to query.
The original authority metric: PageRank
When Google first began, it did have a single authority figure. That was called PageRank, which was all about looking at links to pages. Google counted how many links a page received to help derive a PageRank score for that page.
Google didn’t just reward pages with a lot of links, however. It also tried to calculate how important those links were. A page with a few links from other “important” pages could gain more authority than a page with many links from relatively unremarkable pages.
Even pages with a lot of authority — a lot of PageRank — weren’t guaranteed to rocket to the top of Google’s search results, however. PageRank was only one part of Google’s overall ranking algorithm, the system it uses to list pages in response to particular searches. The actual words within links had a huge impact. The words on the web pages themselves were taken into account. Other factors also played a role.