What is SEO?

As quoted from the SEOmoz.org:

SEO is the active practice of optimizing a web site by improving internal and external aspects in order to increase the traffic the site receives from search engines. Firms that practice SEO can vary; some have a highly specialized focus, while others take a more broad and general approach. Optimizing a web site for search engines can require looking at so many unique elements that many practitioners of SEO (SEOs) consider themselves to be in the broad field of website optimization (since so many of those elements intertwine).

Whereas, based on explanation quoted from wikipedia, that said:

SEO is the process of improving the volume or quality of traffic to a web site or a web page (such as a blog) from search engines via “natural” or un-paid (“organic” or “algorithmic”) search results as opposed to other forms of search engine marketing (SEM) which may deal with paid inclusion. The theory is that the earlier (or higher) a site appears in the search results list, the more visitors it will receive from the search engine. SEO may target different kinds of search, including image search, local search, video search and industry-specific vertical search engines. This gives a web site web presence.