Wiktionary
set-top box
n. A device that connects some source of signal to a television set, often decrypting a digitally encrypted signal from a communications satellite or cable network.
Wikipedia
Set-top box
A set-top box (STB) or set-top unit (STU) is an information appliance device that generally contains a TV-tuner input and displays output to a television set and an external source of signal, turning the source signal into content in a form that can then be displayed on the television screen or other display device. They are used in cable television, satellite television, and over-the-air television systems, as well as other uses.