the process of improving the visibility of a website or a web page in search engines
Search engine optimization (SEO) is the process of improving the visibility of a website or a web page in search engines' "natural," or un-paid ("organic" or "algorithmic"), search results. In general, the earlier (or higher ranked on the search results page), and more frequently a site appears in the search results list, the more visitors it will receive from the search engine's users.
Questions having the seo tag regards the implementation and use of SEO.
Reference: http://en.wikipedia.org/wiki/SEO