Each organic search engine ranking places emphasis on variable factors such as the design and layout, keyword density and the number of relevant sites linking to it. Search engines constantly update and refine their ranking algorithms in order to index the most relevant sites. Other variables that have an impact on search engine placement include the following:
Website owners recognized the value of a high ranking and visibility in search engine results, creating an opportunity for both white hat and black hat SEO practitioners. According to industry analyst Danny Sullivan, the phrase "search engine optimization" probably came into use in 1997. Sullivan credits Bruce Clay as one of the first people to popularize the term. On May 2, 2007, Jason Gambert attempted to trademark the term SEO by convincing the Trademark Office in Arizona that SEO is a "process" involving manipulation of keywords and not a "marketing service."