In the context of display advertising, Cost Per Day (CPD) refers to the cost of an ad campaign that is based on the number of days that the ad is displayed. With a CPD campaign, the advertiser pays a fixed fee for each day that the ad is displayed, regardless of the number of impressions or clicks the ad receives.

CPD campaigns are typically used by advertisers who want to reach a broad audience over a longer period of time, rather than targeting a specific number of impressions or clicks. They are often used for brand awareness campaigns or to promote events or sales.

CPD campaigns can be effective for reaching a large audience, as they allow the advertiser to have their ad displayed for a longer period of time, potentially reaching more people. However, they may not be as effective for driving immediate results, as the cost of the campaign is not based on the number of impressions or clicks the ad receives.