Cost Per Engagement, which is a digital advertising pricing model where advertisers pay for each user engagement with their ad. 

Engagements can include various actions such as clicks, video views, likes, shares, comments, or other interactions depending on the ad format, Cost per engagement, the calculation is the total campaign cost divided by the number if engagements as predefined.  

CPE is a typical pricing model for incentivized (rewarded) inventories. You can learn more about this type of inventory in this article.