Search Engine Optimization (SEO) Definition

Information Science quantifies Search Engines results according to two dimensions:

• Relevancy which is the degree a web pages ranks for the search term. It is visible manifests in terms of the  a search result for a given search phrase.
• Accuracy - of placement for the given search phrase.

SEO is the discipline of maximizing a web site's placement in major search engines for a set of search phrases. Appropriate phrase selection can significantly increase the relevancy of a site to user searches.

From a marketing point of view it is essential that appropriate search terms should lead prospective client to one site and not to its competitors. As search engine page ranking scheme change from time to time and new competitors will try to optimize for competing phrases, one's placement is an ongoing battle with one's peers.

Search Engines claim a moral high ground in obscuring their ranking algorithms, and their penalizing and blacklisting methods. They claim that this practice guarantees the search results that users expect, in terms of accuracy and relevancy. 

However Search engine are provider's of paid listings the search engines and thus have a vested interest in creating a perception of  the value to of paid listing they provide

The methods used in SEO are either internal to the web page or external. For instance placing text into the header is considered internal, but linking to the page from a highly rated web site is external. Another distinction is if a practice is Kosher or likely to get a site banned from a search engine listing. For instance using content only visible to the search engine is considered inappropriate and would get a site black listed.