Target cost per thousand impressions (tCPM): Definition
A way to bid where you set how much on average you’re willing to pay every thousand times your ad is shown (this amount is your target CPM). We'll optimise your bids to help get as many impressions as possible using your Target CPM. Some impressions may cost more or less than your Target CPM, but altogether Google Ads will try to keep your campaign average CPM equal to or lower than the target CPM you set.
Target cost per thousand impressions (tCPM) is available for video campaigns in Google Ads.