Below are definitions from several popular website.
MarketingProfs.com – The altering of a Web site, Web pages and links to Web sites and Web pages to improve visibility, rank and relevance in the organic, crawler-based listings of search engines.
WebsiteNova.com – 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.
Wikipedia.com – A set of methodologies aimed at improving the visibility of a website in search engine listings.
SEOmoz.org – … 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.
Webopedia.com – … the process of increasing the amount of visitors to a Web site by ranking high in the search results of a search engine
But what i know is that good SEO brings in good traffic for our site.