Cost per click (CPC) is the price an advertiser pays to a publisher for each click on their ad. The CPC is calculated with the following formula:
CPC = total cost of campaign ÷ number of clicks
CPC rates mitigate the risk for advertisers as they only pay when their ad actually gets clicked. The cost depends on either targeting high-demand audiences or popular key phrases on Google.
eCPC means effective cost per click and is used to show what the CPC would have been if the advertiser purchased clicks instead of impressions or conversions.
More about cpc on Piwik PRO blog: