Pixel Stuffing is the process of serving one or multiple ads in a single 1X1 pixel frame. Pixel stuffing is typical kind of ad fraud as served ads are invisible to the naked eye.
Pixel stuffing is often done through a 1×1 pixel frame at the bottom of a page.
Pixel stuffing should not be confused with cookie stuffing which is a fraud in affiliate marketing.
For more details and video illustrations of pixel stuffing, see impression stuffing.