A power adapter that contains the plug for the wall outlet. The "wart" is a derogatory reference to the device because it covers up the adjacent socket and renders it unusable. See power adapter
and vampire device
The Wart Is a Hog
Wall warts intrude into the surrounding sockets on a power strip.
The "Wall Wart" Adapter
Many power strips are built with extra space to accommodate the ubiquitous wall wart.