In the world of advertising, the word impressions is used to signal when an ad is shown. It started strictly in online advertising, but has since been used commonly for almost all forms of advertising to describe the number of people who see an ad.
The number of impressions your ad gets, most commonly, is the number of times your ad is shown to a consumer. But impressions can mean different things in different contexts, and there is some debate over how we should count impressions versus how we do it today.
For example, in online advertising, an impression is counted when an ad is displayed. That could mean the ad is displayed on a part of the page that never gets viewed. Or in the case of rotating ads, multiple ads get displayed even though the visitor only sees one or another.
Therefore there is a push in the ad industry to only measure “viewed” or “viewable” impressions. But today, we count every impression the same.
You can also further segment impressions to “unique impressions”. If the same person sees your ad 4 times, that might count as 4 impressions, but only 1 unique impression. Depending on what you’re advertising, you might want to maximize unique impressions rather than just total number, so your ad reaches more people.
With most display advertising, and other ad platforms that are not “cost per click”, advertisers pay based on the number of impressions. So your goal as an advertiser will be to get the most high-quality impressions for your investment, where high-quality might mean viewable, or targeted at the right people to generate the best response (mostly clicks).