Search engine optimization (SEO) is a set of methods aimed at improving the ranking of a website in search engine listings. The term also refers to an industry of consultants that carry out optimization projects on behalf of clients' sites.

Using search engines, visitors can find sites in a variety of ways: via paid-for advertisements in the search engine results pages (SERPs), via third parties who are listed in the search engines, or via "organic" listings, i.e. the results the search engines present users. SEO is primarily concerned with improving the visibility of a site in the organic search results.

High rankings in the organic search results can provide targeted traffic for a site. Obtaining that traffic by other means can potentially be expensive. For particularly competitive terms, the cost per click can run several dollars, or more, when pay per click advertising or banner advertising are used. For even moderately competitive terms the cost can range from a few cents to several tens of dollars per visitor. Given those costs, it often makes sense for site owners to optimize their sites for organic search.

Not all sites have identical goals in mind when they optimize for search engines. Some sites are seeking any and all traffic, and may be optimized to rank highly for common search phrase. This can be a poor marketing strategy for a business because it can generate a large volume of low-quality inquiries that cost money to handle, yet result in little business. The "shotgun approach" to search optimization can possibly work well for a site that has broad interest, such as a periodical, a directory, or site that displays advertising with a CPM revenue model.

Other sites target a specific population, with particular needs or interests. Many businesses try to optimize their sites for large numbers of highly specific keywords that indicate a prospective customer who is ready to buy their product. Focusing on desired traffic can generate more high-quality sales leads, and fewer time-wasting inquiries.