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. In effect, SEO is marketing by appealing to machine algorithms to increase search engine relevance and ultimately web traffic. This is analogous to foot traffic in retail advertising. The term SEO can also refer to "search engine optimizers", an industry of consultants who carry out optimization projects on behalf of clients.