Technically saying the term (SEO) Search Engine Optimization is incorrect as it is not the search engine which is optimises rather we optimise our website so that it can rank higher in the search engine results.

Without going into technical stuff, Search Engine Optimization (SEO) can be defined as the process by which one can increase the volume and quality of trafiic to one's website from search engines like Google, Yahoo etc via "natural search" or "organic" search results by improving its ranking in the search results.