Wiktionary
n. (context marketing English) A form of marketing in which focus is placed on influencers—individuals that have influence over potential buyers—rather than the target market as a whole.
Wikipedia
Influencer marketing (also influence marketing) is a form of marketing that has emerged from a variety of practices and studies, in which focus is placed on specific key individuals (or types of individual) rather than the target market as a whole. It identifies the individuals that have influence over potential buyers, and orients marketing activities around these influencers.
Influencer content may be framed as testimonial advertising where they play the role of a potential buyer themselves, or they may be third parties. These third parties exist either in the supply chain (retailers, manufacturers, etc.) or may be so-called value-added influencers (such as journalists, academics, industry analysts, professional advisers, and so on).
The first approach to that theory comes from a communication classic, The People´s Choice (Lazarsfeld and Katz), a 1940 study on political communication that was also known as Multistep flow model, that claims that the majority of people are influenced by secondhand information and opinion leaders.