CPA is sometimes referred to as "cost per acquisition", which has to do with the fact that many CPA offers by advertisers are about acquiring something (typically new customers by making sales).

Formula to calculate cost per acquisition

Cost per acquisition (CPA) is calculated as: cost divided by the number of acquisitions. So for example, if one spends $150 on a campaign and gets 10 "acquisitions" this would give a cost per acquisition of $15.