Definition of CPC in online advertising
CPC is the Cost per click when buying CPC.
See the definition of a click.
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.
See also definition of eCPC, definition of CPM and definition of RPM.
I also recommended using the CPM calculator to make sure you get your numbers right.