Search Engine Optmization (SEO) — Definition:

The goal of Search Engine Optimization is to increase the number of visitors (a.k.a. "Traffic") that come to a Web Site by developing high ranking in the the Search Engine Results Pages (a.k.a. SERPs). The higher a Web site ranks in the SERPs, the greater the chance that that site will be visited by a user.

It is common practice (e.g. "Human Behavior") for Internet users to not sift through multiple pages of search results, and as a result high ranking is essential for directing more traffic toward the site.

SEO helps to ensure that a site is (1) accessible to search engines and (2) improves the chances that the site will be found by search engines and casual visitors.