A takeover ad unit is an ad unit - or combination of ad units - which "takes control" of a webpage and usually gives high visibility to the advertiser.
Takeover ad is a generic name and may refer to different specific ad units.
Takeover ad units are most often offered on homepages - see homepage takeover for more details and examples - but may also be displayed through run of site.
A page takeover which consists of a combination of two ad units :
Another example of takeover ad :