Page Authority is a score (on a 100-point scale) developed by Moz that predicts how well a specific page will rank on search engines. It is based off data from the Mozscape web index and includes link counts, MozRank, MozTrust, and dozens of other factors. It uses a machine learning model to predictively find an algorithm that best correlates with rankings across the thousands of search results that we predict against.
How is Page Authority Scored?
We score Page Authority on a 100-point, logarithmic scale. Thus, it's easier to grow your score from 20 to 30 than it would be to grow from 70 to 80. We constantly update the algorithm used to calculate Page Authority, so you may see your score fluctuate from time to time.
Page Authority vs. Domain Authority
Whereas Page Authority measures the predictive ranking strength of a single page, Domain Authority measures the strength of entire domains or subdomains. The same is true for metrics such as MozRank and MozTrust.
Where can you find Page Authority?
Technical Definition of Page Authority
Page Authority is Moz's calculated metric for how well a given webpage is likely to rank in Google.com's search results. It is based off of the Mozscape web index and includes link counts, mozRank, mozTrust, and dozens more. It uses a machine learning model to predicatively find an algorithm that best correlates with rankings across thousands of search results that we predict against.
It's best to use Page Authority (PA) and Domain Authority (DA) as comparative metrics when doing research in the search results and determining which sites/pages may have more powerful/important link profiles than another. While more specific metrics like mozRank can answer questions of raw link popularity and link counts can show the raw quantities of pages/sites linking, the authority numbers are high level metrics that attempt to answer the question "How strong are this page's/site's links in terms of helping them rank for queries in Google.com?"
SEO Best Practice
How do I influence this metric?
Unlike other SEO metrics, Page Authority is difficult to influence directly. It is made up of an aggregate of metrics (MozRank,MozTrust, link profile, and more) that each have an impact on this score. This was done intentionally; this metric is meant to approximate how competitive a given site is in Google.com. Since Google takes a lot of factors into account, a metric that tries to calculate it must incorporate a lot of factors, as well.
The best way to influence this metric is to improve your overall SEO. In particular, you should focus on your link profile—which influences MozRank and MozTrust—by getting more links from other well-linked-to pages.
In this video, Rand Fishkin describes the importance and details of Domain Authority.
Moz's comprehensive guide to the practice of search engine optimization for those who are unfamiliar with the subject.