What you pay for one ad click. The most basic paid-traffic metric.
CPC = ad spend ÷ clicks. Depends on platform, competition, and targeting. Search ads (Google) run $1-$20 CPC depending on keyword. Social ads (Meta, TikTok) run $0.30-$3 typical. Display is $0.10-$1. CPC isn't the goal, it's a lever. A cheap CPC on bad traffic is worse than an expensive CPC on great traffic.
Know your CPC per channel so you can reason about scale. But don't obsess over it; the real questions are CAC and ROAS. Cheap clicks that don't convert are worse than expensive clicks that do.