What is an Impression?
Impression refers to ad impression; the number of times an ad is shown. An impression is sometimes just called an ad view.
This term was started during the era when advertising was all about magazines and newspapers, and an impression was a literal impression of ink on a page.
Does Google Adsense pay for an impression?
Google Adsense, although it is a CPC based advertising network, has something called Cost Per Mille (CPM). In this model, users pay for a fixed amount of money for 1000 impressions.