Search engine optimisation (SEO) is a subset of search engine marketing and seeks to improve the number and quality of visitors to a web site from "natural" ("organic" or "algorithmic") search results.

The quality of visitor traffic can be measured by how often visitors, using specific keyword or keyword phrase, leads to a desired conversion action, such as making a purchase or requesting further information.

In effect, SEO involves presenting appealing, interesting and relevant content to human visitors first, and secondly, the content constructed in a way that 'appeals' to search engine spiders and machine algorithms to increase search engine search relevance.

The term SEO can also refer to "search engine optimisers", an industry of consultants who carry out optimization projects on behalf of clients.