Final answer:
An ad impression in web marketing refers to a single display of an advertisement to a user, which is a metric used to measure the reach of an ad.
Step-by-step explanation:
In the context of Web marketing, the term ad impression refers to one user viewing one ad. This metric is crucial for advertisers and publishers as it indicates the number of times an ad was displayed, regardless of whether it was clicked or not. Not to be confused with ad clicks, which measure user interaction, an ad impression is counted each time the ad is fetched and potentially seen by a user. Therefore, it is a measure of the reach of the ad