Basically any time an ad is fetched (from the server/etc.) and displayed to a visitor or on a web page that is counted as an impression. The more impressions then the more times that particular advertisement/image/etc. was displayed on a web page.
Hi: An impression is when your advertisement is shown to a prospective customer. Each time your advertisement is loaded on someones website, forum, or wherever you are advertising this is considered a impression. It doesn't matter if the person clicks on your ad or not. They saw it or it at least was presented for them to see. Each time your advertiser sends out your ad and it reaches the site it's an impression. Good Luck, Dan
As everyone on this thread has described in one form or another, an impression is simply the appearance of your advertisement on any one of websites, blog sites, product review sites and so on. Some traffic generation vendors will charge you for the number of impressions (appearances) your advertisement receives. But a lot of good information here in the assorted posts above!
Impression, sometimes called a view or an ad view, is a term that refers to the point in which an ad is viewed once by a visitor, or displayed once on a web page. The number of impressions of a particular advertisement is determined by the number of times the particular page is located and loaded.