Search engine marketing (SEM) is a form of Internet marketing that involves the promotion of websites by increasing their visibility in search engine results pages (SERPs) through optimization (both on-page and off-page) as well as through advertising (paid placements, contextual advertising, and paid inclusions). – Wikipedia
I think Search Engine Marketing (SEM) is a lot more refined and is specific to buying visibility, in advertisement areas, on Search Engine Results Pages (SERP).
Search Engine Marketing, Google and Ad Placement
According to Hits Link and Stat Owl, Google’s Search Engine has the single largest market share, by almost 10 times the other search engines. While you don’t want to ignore the other search engines altogether, Google gets used the most, so it will provide the most visibility.
If you’ve ever run a search on Google, the image above is a familiar site. It is a Search Engine Results Page (SERP), highlighting the Ad Placements of paid advertisements. If Search Engine Marketing is buying visibility, in advertisement areas, on Search Engine Results Pages, the picture above, is the end result.