One of the easiest things to trip up on in digital marketing is the word “Impression” because it can mean so many things. It always means *roughly* the same thing, but *roughly* doesn’t cut it if you are serious about what you are doing.
A Page Impression is different from an Ad Impression, which is not exactly the same as a Viewable Impression. There are Premium Impressions (although not really), and Impression Shares, Post Impressions and things that happen Post-Impression. There are Viral Impressions, Unique Impressions, Top Impressions and Absolute Top Impressions.
Impressions are a simple idea – a way to count when something successfully loads a single time. It’s just that the term impression is used in so many different ways that it’s easy to get a bit lost when people talk about them.
Example: Page Impressions vs Ad Impressions
The best example of how impressions are simple yet complicated is the difference between page impressions and ad impressions. A Page Impression is counted every time a page is loaded. An Ad Impression is counted every time an ad is loaded.
So would a webpage with ads on it have the same number of ad impressions as page impressions? No! Because webpages have multiple ads on them, so for every page impression there are multiple ad impressions.
So can you simply multiply the number of ads on a page by the number of page impressions to get the number of ad impressions? No! Because digital marketing discrepancies *always* happen, and so not every loaded webpage will include a complete set of loaded ads.
Every single person working in display advertising has been tripped up by this not-clear-cut distinction more than once (I guarantee it). In fact, explaining this difference is one of the reasons why I started this site in the first place (along with explaining what MPU stands for).