vCPM, or Cost Per Mille Viewable Impressions, is a pricing model used in online advertising that refers to the cost an advertiser is willing to pay for 1,000 viewable impressions of their ad. Viewable impressions refer to the number of times an ad is served to users and is actually seen by them. vCPM is typically calculated on a per-thousand basis and is used to determine the cost of an ad campaign.

Unlike CPM, which measures the total number of impressions regardless of whether the user sees the ad, vCPM only counts impressions that are actually viewable. This is important because it allows advertisers to pay only for ads that have a higher chance of being seen by users, rather than paying for impressions that may not be noticed.

vCPM is often used in conjunction with other metrics, such as click-through rate (CTR), to measure the effectiveness of an ad campaign and determine its profitability. By comparing the vCPM to the CTR, advertisers can determine the return on investment (ROI) of their ad spend and make adjustments as needed.