CPC (cost per click)
CPC is the average cost paid for each click, the dominant pricing model in search and common across social. It prices interest rather than delivery, but a click is still an intermediate step - CPC only matters in combination with what happens after the click.
In practice
A cheap click that never converts is more expensive than a dear one that does. CPC belongs alongside conversion rate and CPA, not in place of them.
Rising CPCs signal auction pressure: more competitors, higher bids or declining quality signals. Diagnose before simply paying more.