Search engine optimization (SEO), considered by many to be a subset of search engine marketing, is a term used to describe a process of improving the volume or quality of traffic to a web site from search engines, usually in "natural" ("organic" or "algorithmic") search results. Those efforts may also be seen in more narrow vertical search engines involving areas such as local search. Many site owners and consultants engaging in SEO attempt to pursue qualified visitors to a site, and the quality of visitor traffic can be measured by how often a visitor using a specific keyword phrase leads to a desired conversion action, such as making a purchase, viewing or downloading a certain page, requesting further information, signing up for a newsletter, or taking some other specific action.

In a broad sense, SEO is marketing by understanding how search algorithms work and what human visitors might search for, to help match those visitors with sites offering what they are interested in finding. Creating web pages with SEO in mind does not necessarily mean creating content more favorable to algorithms than human visitors. Some SEO efforts may involve optimizing a site's coding, presentation, and structure, without making very noticeable changes to human visitors, such as incorporating a clear hierarchical structure to a site, and avoiding or fixing problems that might keep search engine indexing programs from fully spidering a site. Other, more noticeable efforts, involve including unique content on pages that can be easily indexed and extracted from those pages by search engines while also appealing to human visitors.

The term 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 of site owners who may perform SEO services in-house.

Search engine optimizers often offer SEO as a stand-alone service or as a part of a larger marketing campaign. Because effective SEO can require making changes to the source code of a site, it is often very helpful when incorporated into the initial development and design of a site, leading to the use of the term "Search Engine Friendly" to describe designs, menus, content management systems and shopping carts that can be optimized easily and effectively.