Impression stuffing is the process of inserting many invisible ads on a webpage.
Impression stuffing can be done through pixel stuffing whereby one or many 1X1 pixel frames are placed all over a page and trigger ad impressions which are invisible to the naked eye. In some cases, many ads are served in a single 1X1 pixel frame.
An other type of impression stuffing is to layer multiple ads on top of each other in a legitimate ad slot. Then, only the one on top gets seen and those below are not viewed at all. This form of impression stuffing is called ad stacking.
Some examples of impression stuffing :