In online advertising you either pay by click (CPC = cost per click) or impressions (CPM = Cost Per Thousand, “M” being the Roman numeral for thousand. This method has been around for a long time).
But what is an online impression?
There are “served impressions”, the number of times the ad server sends out your ad, and “viewed impressions”. Obviously as an advertiser you’re only interested in the latter. Facebook uses viewed impressions which it defines as “We measure an ad impression the moment an ad enters the screen of a desktop browser or mobile app.” But what if they only see part of your ad, or what if they only see it for a fraction of a second? Are those worth paying for?
What if “they” are a bot? Some studies put bots at close to half of all Web traffic.
Google has this definition for an impression:
When buying ad space in a newsletter or other publication on a CPM basis you should find out if it’s served or viewed impressions.
There are arguments for both CPC and CPM, and which you use to buy your ads will depend on your goals.