What is Page Authority?

Page Authority predicts the likelihood of a single page to rank well, regardless of its content. The higher the Page Authority, the greater the potential for that individual page to rank well in search results.
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 fluxuate from time to time.

Technical Definition of Page Authority

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Page Authority is SEOmoz'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 Linkscape 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?"