Well, it’s a short form of Cost Per Click. This amount is calculated as you earn each time a visitor of your site clicks on your ad. The CPC is determined by the advertiser who are willing to pay for advertising on your site through Google. Mostly it will be CPA which is nothing but Cost-per-Action. That is, unless your visitor does some particular action as mentioned by the advertiser terms, you will not get higher returns on Google Adsense income.