Definition of CPM in online advertising
CPM, or Cost per mille, is cost per thousand advertising impressions in online advertising. Mille is the latin word for thousand.
From a publishers point of view, CPM is the basic currency for all of their inventory. CPM is what the publisher should optimize.
The reason why we want the coust per thousand impression, instead of the cost per single impression is simple:
The cost for each single impression would be a very small number.
So instead of having a cost per single impression presented as 0.0078, we have the cost per thousand impression presented as 7.8.
The CPM is actually about real money, so it should be noted as USD, EUR or some other currency. For example 10 USD CPM. See the definition of Cost
Another related metric is RPM: Revenue per thousand impressions.
And of course, let's not forget how to calculate the CPM.
I also recommended using the CPM calculator to make sure you get your numbers right.