Search Engine Optimization (SEO) is the practice of increasing the visibility of a web page or a website in a web search engine’s unpaid results—often referred to as “natural”, “organic”, or “earned” results. In general, the earlier (or higher ranked on the search results page), and more frequently a site appears in the search results list, the more visitors it will receive from the search engine’s users; these visitors can then be converted into customers. SEO may target different kinds of search, including image search, video search, academic research search, news search, etc. Many websites focus on improving their ranking in major search engines such as Google and Bing, though some websites focus their SEO strategies on less-known search engines such as DuckDuckGo. As of June 2009, SEM was considered a mainstream marketing technique used by advertisers.