Page Authority (PA) is a metric developed by Moz to predict how well a specific page will rank on search engine result pages (SERP). It scores pages on a 100-point scale, with higher scores indicating a greater ability to rank. Page Authority is calculated using multiple factors, including linking root domains and the number of total links, into a single PA score. This score can be used to compare the strength of individual pages and track the ranking ability of a page over time. However, it's important to note that PA is a relative tool, best used to compare pages within a specific website rather than across different sites.