Invalid traffic & fraud
Ad stacking
Ad stacking is piling several ads on top of each other in one ad slot, so only the top ad is visible but every ad in the stack is counted and billed.
A single space on a page or in an app should show one ad at a time. With ad stacking, a publisher or fraudulent script loads many ads into the same space, piled on top of one another. The user sees only the top one, but all of them register impressions and, sometimes, clicks.
It can be deliberate fraud, or a side effect of badly built pages or ad mediation setups that keep loading ads without clearing the old ones. Either way, the hidden ads had no chance to be seen, so the MRC IVT guidelines classify them as invalid; they belong with pixel stuffing and other hidden ads.
Viewability measurement catches much of it, because the stacked ads underneath register as obscured or zero-percent viewable. Verification scripts can also spot many ad containers occupying the same screen coordinates. A tell-tale sign for advertisers is lots of served impressions with very low viewability and near-zero clicks from a specific placement.
Think of it like this
Ad stacking is like gluing ten posters on top of each other on one billboard and charging all ten advertisers for the week.
An example
A mobile game's banner placement reports 12 impressions for every visible refresh. A verification vendor finds 11 ads rendered beneath the top one, with 0% viewability, and flags the placement.
Related terms
Pixel stuffing
Pixel stuffing is squeezing an ad, or a whole page of ads, into a tiny frame, often 1x1 pixel, so it counts as served but no one can see it.
Hidden ads
Hidden ads are ads that load and count as impressions but are placed where no one can see them, such as off-screen, behind other content, stacked or shrunk to tiny sizes.
Viewability
Viewability measures whether an ad actually had a chance to be seen on screen; the MRC standard is 50% of pixels in view for one second (display) or two seconds (video).
Ad fraud
Ad fraud is the deliberate faking or misrepresenting of ad impressions, clicks, installs or conversions so that someone gets paid for advertising that no real, interested human saw or did.
SIVT (sophisticated invalid traffic)
SIVT (sophisticated invalid traffic) is invalid traffic designed to look human or legitimate, which can only be found through advanced analytics, multi-point corroboration and often human review.
Sources: MRC Invalid Traffic Detection and Filtration Guidelines Addendum (2020 update), MRC Viewable Ad Impression Measurement Guidelines