Cost-per-thousand impressions (CPM): Definition
A way to bid where you pay per one thousand views (impressions) on the Google Display Network.
Viewable CPM bidding ensures that you only pay when your ads can be seen. Existing CPM bids will be converted to vCPM automatically, but it's best to update your bids since viewable impressions are potentially more valuable. Learn more about using viewable CPM bids.