In digital advertising, what does the term 'impression' refer to?

In digital advertising, the term 'impression' specifically refers to the number of times an ad is viewed, making the correct answer the measurement of visibility or exposure that an advertisement receives. Each instance of the ad being loaded and displayed on a user's screen counts as one impression, regardless of whether the user interacts with the ad or not. This metric is crucial for advertisers as it helps them understand how often their ads are seen by potential customers, which can inform decisions about ad placement, targeting, and overall campaign effectiveness.

The other options pertain to different concepts in digital advertising. The number of clicks on an ad measures engagement and indicates user interest but does not encompass the full scope of ad visibility. The total cost of an ad campaign is related to budgeting and financial considerations but does not reflect how often ads are seen. Lastly, return on investment (ROI) assesses the effectiveness of an ad campaign in terms of the profit generated relative to its cost, which is a different performance evaluation than simply counting impressions.