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 optimize your bids to help get as many impressions as possible using your target CPM (tCPM). Some impressions may cost more or less than your tCPM, but altogether Google Ads will try to keep your campaign's average CPM equal to or lower than the tCPM you set.
Target cost-per-thousand impressions is available for video campaigns in Google Ads. For video ad sequence campaigns, tCPM optimizes for completions instead of impressions, but works the same as other video campaign types.