The ability of a page or domain to rank well in search engines. Five large factors associated with site and page authority are link equity, site age, traffic trends, site history, and publishing unique original quality content.
Search engines constantly tweak their algorithms to try to balance relevancy algorithms based on topical authority and overall authority across the entire web. Sites may be considered topical authorities or general authorities. For example, Wikipedia and DMOZ are considered broad general authority sites. This site is a topical authority on SEO, but not a broad general authority.