A single instance of an online advertisement being displayed.*
* This definition may be an over-simplification, as there is no standard way to count impressions. All of the differences can add up to very large discrepancies, yet people make purchases based on impression every day.
So… what is the definition?
Currently, whatever a buyer and seller agree on.
see also: Digital Marketing Tools
Display Advertising Stats 2019: SEMrush Study
SEMrush (May 3, 2019) sponsored
Why Can’t Johnny Count Ad Impressions?
ClickZ (September 16, 2002)
Why We Can’t Kill the Impression Just Yet
ClickZ (November 29, 2001)
The Impression Must Be Destroyed! (Part 2)
ClickZ (November 27, 2001)
The Impression Must Be Destroyed! (Part 1)
ClickZ (November 20, 2001)
That Sneaking Impression: Part 2
ClickZ (August 11, 2000)
That Sneaking Impression
ClickZ (August 4, 2000)