A process employed to increase a website's traffic by boosting the visibility of this site in the unpaid results of search engine (such as Google, Yahoo, Bing). If a consumer searches something and in the results of that search the website appears high-up, this will lead to more visitors at the site in question. Taken into consideration in this process is: how a search engine functions, what is being searched, keywords that will be typed, and which search engine is preferred by the target audience. All kinds of searches can be targeted such as image search, video search, local search, academic search, and news search.

Source: searchengineland.com/guide/what-is-seo