What it means
A9 is the name of Amazon's search algorithm — named after Amazon's former search subsidiary A9.com ('A' plus nine more letters of 'algorithms'), not after a number of ranking factors. A9 decides which products appear in which order for a search, weighing two dimensions: relevance (do the search terms match the listing's title, bullets, and backend keywords?) and performance (how often the product actually sold after this search — influenced by price, reviews, availability, and fulfillment method). Without sales, a product barely climbs organically, no matter how good the keywords are.