Credibility
This section of the manual talks a lot about "credibility" and "authority". Essentially these factors can be described as:
Those sites that have a good reputation within their specific field / s, and are cited often as being knowledgeable about their chosen sphere.
i.e. "credibility" and "authority" are about a site having existed for several years, featuring lots of useful and unique content, and having multiple inbound links from other relevant, quality sites.
Like Page Rank, these factors are almost self-perpetuating, but they are important when determining who to try and achieve an inbound link from, and who to give an outbound link to.
