Hard to define as it’s a skill-set that relies on a wide ranging set of sub skills, the art of seo, at its heart, is an understanding of how people use search engines, and it’s an understanding of how search engines generate their natural search engine results pages (serps), on NAVIGATIONAL, INFORMATIONAL (the VAST majority of activity) and TRANSACTIONAL keyword queries. It’s an understanding of the short term and long term risks involved, and an understanding of the type of content Google (especially) WANTS to return in it’s natural serps.