In marketing, an individual or group that has a significant amount of influence on the opinions or habits of their fans or followers, perhaps especially their purchasing habits. A lot of companies try to market directly to so-called influencers—bloggers and vloggers, mostly—in the hopes that the influencer will boost sales by promoting the product among their followers. You have to wonder how many of these influencers are really just paid shills.

In marketing, an individual or group that has a significant amount of influence on the opinions or habits of their fans or followers, specifically as it relates to health and medicine. The writer, a patient influencer on social media, mainly posts about her struggles with a chronic illness. He's a patient influencer who beat cancer and now attends medical conferences and advocates on behalf of patient care.