My definition of SEO is the combination of techniques you using to make a website rank high on search engines.

This is the definition I give to my job interview four years ago in Kent but after a couple years and discuss with different clients I have to be more specific. Because search engine optimization job is more complexes than that, you need to use a lot of different techniques. My definition of SEO could be also: the process,of building, creating, designing or updating a site, even content, with the goals of increasing visibility within search engines, and get the top rank on search engine results pages, for a desired market segment.