CPA (Cost Per Acquisition) measures the cost to acquire something that is not a customer — for example, a registration, activated user, trial, or a lead.

Customer acquisition cost (CAC) is related to CPA. Cost Per Acquisition is usually used to measure the cost of things that are leading indicators to CAC.

CAC specifically measures the cost of acquiring a customer.

Customer acquisition cost (CAC) and cost per acquisition (CPA) are commonly conflated, and yet in reality they're completely different metrics. Understanding the difference is the start to understanding CAC in depth.