Tough question, "What is SEO?". As with pretty much all internet-related terms, concepts and notions, that of "SEO" does not have a unique definition and it is a blurry concept in most people's mind.

Still, what is SEO? Since there is no ultimate, fully comprehensible definition for "SEO", the only way to go is to take a look at several definitions and try to merge them in order to have the right perspective.

Wikipedia : "Search engine optimization (SEO) is a set of methodologies aimed at improving the visibility of a website in search engine listings. The term also refers to an industry of consultants that carry out optimization projects on behalf of client sites."

Fakezilla : "The changes that are made to the content and code of a web site in order to increase its rankings in the results pages of search engines and directories. These changes may involve rewriting body copy, altering Title or Meta tags, removal of Frames or Flash content, and the seeking of incoming links."

The Web Search Workshop : "The term used to describe the marketing technique of preparing a website to enhance its chances of being ranked in the top results of a search engine once a relevant search is undertaken. A number of factors are important when optimizing a website, including the content and structure of the website's copy and page layout, the HTML meta-tags and the submission process."

6am Media : "The process of improving web pages so that it ranks higher in search engine for targeted keywords with the ultimate goal of generating more revenue from the web site. There are many SEO techniques. In general, these techniques can be categorized as On-Page Optimization, On-Site Optimization, and Off-Site Optimization. There are also two schools of SEO: white hat SEO and black hat SEO. White hat SEOs are those that play by the rule (actually guidelines provided by search engines). Black hat SEOs are those that push the limit of SEOs and employ some questionable or prohibited techniques (according to the guidelines). These black hat SEO techniques are also commonly known as spam."

Website NOVA : "acronym for search engine optimization. This is the process of making a website 'search-engine-friendly. Search engine optimization is primarily used to increase rankings in SERPs, and effective SEO can increase the potential of your website and bring in more traffic."

Thousands more definitions are available, almost as many "SEO guru's" you will find online ("The Guru Problem" is actually the title of another article to be published soon).
As you can see, no definition is like another, but they all tend to converge to a certain common understanding.

There are numerous techniques and tools used to achieve SEO goals, and they should NOT be included within a definition.

Since it is not correct to define a concept through its tools, here is a definition I have come up with after long deliberations. The suggestion is to define SEO as follows:

SEO = abbreviation for "Search Engine Optimization", the process of optimizing and tuning a web site and gaining online awareness for it, in order to deliver targeted visitors and ensure high conversion rates.

When done correctly, SEO activities must:
- make search engines crawl the site;
- make search engines index the site;
- ensure a high ranking among SERPs (Search Engine Results Pages) for given keywords;
- achieve a high page rank;
- drive targeted traffic;
- achieve high conversion rates among the site's visitors.