CPM refers to cost per thousand, or cost to reach one thousand units (people, households, circulation, impressions, etc.) via a given advertising outlet or medium. PCI, or per column inch, is how print publications measure their display ads. CPC stands for cost per click: the amount of money an advertiser pays when a user clicks their ad once. These tools are used in marketing to create a benchmark to calculate relative cost of an advertising campaign or an ad message in a given medium.