Cost-per-engagement (CPE) bidding, also referred to as engagement-based pricing, is an internet advertising model where advertisers only pay when users actively engage with ads.
Examples
Advertisers can pay for lightbox ads (a type of expandable ad that can expand to a very large size) on a CPE basis, which means that publishers generate earnings from lightbox ads when users choose to engage with the ads, e.g., by hovering over them for two seconds to expand the ads.
Another example of engagement-based pricing allows advertisers to pay only when the user clicks on an ad and spends more than 15 seconds reading the landing page.
References
Cost-per-engagement Wikipedia(Text) CC BY-SA