What is an essential characteristic of an ad impression?

An essential characteristic of an ad impression is that it measures the visibility of the advertisement. When an ad impression is counted, it indicates that the ad has been served and potentially viewed by a user, regardless of their interaction or engagement with the ad. This metric is critical for advertisers and marketers to evaluate the reach of their campaigns and to ensure that their ads are being displayed in front of an audience. The visibility aspect helps in assessing the effectiveness of ad placements and strategies in capturing consumer attention.

Other options focus on different aspects of advertising that do not align with the foundational definition of an ad impression. While user action, cost representation, and engagement metrics are important for measuring the overall performance of an advertising campaign, they are not defining characteristics of an ad impression itself, which fundamentally revolves around visibility and serving of the ad.