Definition of CPC in online advertising
CPC is the Cost per click when buying CPC.
When buying CPC from an ad network, the network usually converts the CPC bid to CPM (or RPM) in the background, via the historical or predicted Click-through rate, so the network can optimize their CPM. The ad networks inventory is after all ad impressions, which they want to get most out of.
In order to get a relatively high volume of ad impressions when buying CPC you need to make sure the ads is relevant and inviting to be clicked on. Low click-trough rates will make the ad network anticipate a low CPM for your campaign, which in turn gives the ad network a low revenue per impression, and they are likely to prioritize other competing campaigns.
A similar metric is PPC, or Pay per click.