Impression
An advertisement's (usually a banner ad) appearance on a web page. Ad space is often sold on a CPM basis.

Investopedia Says:
For example, if the page you're on shows five ads, that's five impressions. On this particular page there are two impressions, one for the banner at the top and one for the banner on the bottom of the page.

Related Terms:
Banner Advertising
Click Through Rates
CPC
CPM

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