What is Search Engine Optimization?
SEO stands for Search Engine Optimization. It is a process, in which a web page or a web site is optimized for search engines. By optimization, a search engine such as Google finds the page, includes it in the index, and displays the page when someone is searching for information.

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From the webopedia.com Source
Short for search engine optimization, the process of increasing the amount of visitors to a Web site by ranking high in the search results of a search engine. The higher a Web site ranks in the results of a search, the greater the chance that that site will be visited by a user. It is common practice for Internet users to not click through pages and pages of search results, so where a site ranks in a search is essential for directing more traffic toward the site.
SEO helps to ensure that a site is accessible to a search engine and improves the chances that the site will be found by the search engine.