Cost per Install. It's the cost for each app download that you are paying. It's the same as CAC, but for mobile apps it's called CPI.
This number is usually giveng to you by the platofmr that you are using to do paid advertising (like Google or Facebook) or you can calculate it by divind your monthly budget by the numbers of downloads that you had in a month. Be sure to divide by the number of total downloads not by the number of new active user.