What is SEM derived demand ?

SEM is the synthesis of many online marketing methods, to help put your site in the desired position in the search results on the Internet, apply to search engines like Google, Bing, Yahoo ...
SEM consists of forms: SEO and PPC.

Derived demand is an economic term describing the demand for a good/service resulting from the demand for an intermediate or related good/service. It is a demand for some physical or intangible thing where a market exists for both related goods and services in question.