Here are the definitions of SEO or Search Engine Optimization:
[1] Is the process of improving the volume or quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") search results. Typically, the earlier 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, and industry-specific vertical search engines. [Source]
[2] SEO is an acronym for "search engine optimization" or "search engine optimizer." Deciding to hire an SEO is a big decision that can potentially improve your site and save time, but you can also risk damage to your site and reputation. [Source]
[3] Search engine optimization means ensuring that your Web pages are accessible to search engines and are focused in ways that help improve the chances they will be found. [Source]