Search engine optimization (SEO) is the process of improving the volume and quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") search results. Usually, the earlier a website is presented in the search engine's 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, and industry-specific vertical search engines.

As a marketing strategy for increasing a website's relevance, search engine optimization considers how search algorithms work and for what people search. Search engine optimization efforts may involve a site's coding, presentation, and structure, as well as fixing problems that could prevent search engine indexing programs from fully "spidering" (exploring) a site. Other, more noticeable efforts may include adding unique content to a website, ensuring that content is easily indexed by search engine robots, and making the site more appealing to visitors. Another class of techniques, known as "Black hat" SEO or spamdexing, use methods such as link farms (sites made specifically for deceptively increasing the popularity of a website) and keyword stuffing that tend to harm search engine user experience. Search engines look for sites that employ these techniques and may remove their listings.

The initialism "SEO" can also refer to "search engine optimizers", a term adopted by an industry of consultants who carry out optimization projects on behalf of clients, and 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 and design. The term "search engine friendly" may be used to describe web site designs, menus, content management systems and shopping carts that are easy to optimize.