What you paid for one conversion, whatever 'conversion' you defined.
CPA is ad spend divided by conversions. The conversion definition matters: CPA for a lead is different from CPA for a paying customer is different from CPA for a trial signup. Be specific about what 'acquisition' means. Often used interchangeably with CAC, but strictly CPA is per-conversion (any conversion event) while CAC is specifically per-customer.
CPA is useful at the campaign level when you're optimizing top-of-funnel. Compare CPA across campaigns to find the cheapest source of leads. But never optimize CPA without checking whether those leads convert to customers downstream.