Extended definition
Page-level equivalent for DA. Also Moz score, unofficial.
Context and application
PA measures the authority of a specific page, not the entire domain. The difference is relevant for targeted backlinks: a backlink from homepage (high PA) vs a backlink from a comment (low PA). Ahrefs equivalent: UR (URL Rating). On large sites, pillar pages typically have much higher PA than support pages — this reflects the correct structure of internal linking + targeted external backlinks to pillar.