In online advertising, there are two popular methods of billing. One is cost per click and another is cost per impression. Cost per click (CPC) bills by the number of times the ads is clicked while cost per impression (CPI) bills by the number of times the ads is seen. Usually, when using cost per impression, advertisers will be charged every 1000 impressions. The term cost per mille (CPM) is used instead as the advertising industry is more familiar with it.