Definition of 'Impression'
A metric used to quantify the display of an advertisement on a web page. Impressions are used in banner advertising, which often pays on a per impression basis. Frequently, these are measured by Cost Per Mille (CPM), where mille refers to 1,000 impressions.
Investopedia explains 'Impression'
For example, a banner ad might have a CPM (cost per thousand) of $5, meaning that the website owner receives $5 every time an ad on his website is displayed 1,000 times.
The owner of a website may be paid for each ad impression. Other advertising arrangements may only pay the website owner when a visitor clicks on the ad, or clicks on the ad and makes a purchase. Typically, advertisers pay less for an ad campaign based solely on impressions and more for campaigns based on click-throughs and conversions. The reason for this difference in pay rates is that an ad that causes its viewer to take action resulting in a sale, is more valuable to the advertiser than one that does not.