CPA is an acronym for Cost per Acquisition or Cost per Action. 

CPA is a cost model where the app advertiser pays the ad publisher a fixed rate when a user completes a predefined post-install event, such as a purchase or registration. The calculation is the total advertising cost divided by the number of event that occurred  

While CPA sounds like a “no risk” cost model, CPA has lead to a significant rise of ad fraud, and attribution fraud.