Strictly speaking, a Voken ® ad unit is a rich media ad unit developed by eyeReturn Marketing company.
A Voken is a "free- floating" ad unit, served in a transparent layer over the page which loads automatically without any user interaction. Duration maximum is 8 or 10 seconds at which time the ad closes automatically. A Voken most often runs in conjunction with another ad placement.
In a broader sense, voken is often used as a synonym for ad units also known as OTPs (over the top), floating ads or top layers.