The Definition Of Impressions
The amount of impressions is the amount of times that an advertisement has been shown (through Google AdWords or other forms of PPC advertising) or a webpage has been listed on a SERP (Search Engine Results Page, in the case of search engine optimization).
Note that the amount of impressions differs from the amount of times that an advertisement is actually seen. Impressions can be shown “under the fold”. This means that visitors have to scroll down to be able to see the advertisement. This significantly increases the amount of advertisements that are never seen by a potential customer.
Within Google Adwords you are able to exclude impressions that are shown under the fold. This is often recommended by Google AdWords professionals, especially for CPM-campaigns.