Cost per acquisition (CPA) is the average ad spend to produce one conversion, calculated as spend divided by conversions. It is the metric, distinct from Target CPA, which is a Smart Bidding strategy that aims for a CPA you set.
The metric versus the target
CPA the metric is spend ÷ conversions for whatever you counted as a conversion. Target CPA is the bidding strategy that tells Google to chase that number. The catch is that CPA is only as meaningful as the conversion behind it: a cheap CPA on a shallow event like a form fill can hide an expensive CAC.
CPA versus CAC
People use CPA and CAC loosely, but they are not the same. CPA usually counts a platform conversion (a lead, a signup); CAC is the full cost to win a paying customer. Optimize to a CPA on the wrong event and you can scale cost while starving revenue, which is why we tie CPA back to the qualified end of the funnel.