Search engine optimization (SEO) is a subset of search engine marketing, and deals with improving the number and/or quality of visitors to a web site from "natural" (aka "organic" or "algorithmic" search engine) listings. The term SEO can also refer to "search engine optimizers", an industry of consultants who carry out optimization projects on behalf of clients.

Search engines display different kinds of listings on a search engine results page (SERP), including paid advertising in the form of pay per click advertisements and paid inclusion listings, as well as unpaid organic search results and keywords specific listings, such as news stories, definitions, map locations, and images. SEO is concerned with improving the number and position of a site's listings in the organic search results.

SEO strategies vary widely, in accordance with the specific site. Broadly speaking, SEO may be geared towards increasing either, or both, the total number and quality of visitors from Search Engines. The quality of a visitor can be measured by how often a visitor using a specific keyword leads to a desired conversion action, such as making a purchase or requesting further information.