Cost Per Impression is the cost of your marketing campaign divided by the number of impressions generated through that specific campaign. Usually the impressions are counted based on the number of times an ad appears on a website or through a network of websites.

For example:

$1,500 (spent on Adwords)
___________________     =  $0.015 CPI
100,000 impressions

Other methods of measuring the success of a digital marketing campaign: