Final answer:
The term used to describe the number of times a user sees an ad is called an impression.
Step-by-step explanation:
The term used to describe the number of times a unique user sees an ad over a given time period is impression. An impression is counted each time an ad is served to a user's screen, regardless of whether the user interacts with the ad or not. For example, if a user sees the same ad three times, it would be counted as three impressions.