Search engine optimization (SEO) is the process of improving the volume & quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") search results for targeted keywords. Usually, the earlier a site is presented in the search results or the higher it "ranks", the more searchers will visit that site.  SEO can also target different kinds of search, including image search, local search & industry-specific vertical search engines. As a marketing strategy for increasing a site's relevance, SEO considers how search algorithms work & what people search for.

SEO efforts may involve a site's coding, presentation & structure, as well as fixing problems that could prevent search engine indexing programs from fully spidering a site. Other, more noticeable efforts may include adding unique content to a site, ensuring that content is easily indexed by search engine robots & making the site more appealing to users. Another class of techniques, known as black hat SEO or spamdexing use methods such as link farms & keyword stuffing that tend to harm search engine user experience. Search engines look for sites that employ these techniques & may remove them from their indices.

The initialism "SEO" can also refer to "search engine optimizers" or "Search Engine Optimization", terms adopted by an industry of consultants who carry out optimization projects on behalf of clients & by employees who perform SEO services in-house. Search engine optimizers may offer SEO as a stand-alone service or as a part of a broader marketing campaign. Because effective SEO may require changes to the HTML source code of a site, SEO tactics may be incorporated into web site development & design. The term "search engine friendly" may be used to describe web site designs, menus, content management systems, URLs & shopping carts that are easy to optimize.