From Longman Dictionary of Contemporary Englishlistening deviceˈlistening deˌvice noun [countable] TSECRETa piece of equipment that allows you to listen secretly to other people’s conversations SYN bug
Examples from the Corpus
listening device• I took the liberty of planting a listening device in the jukebox you ... earned.• The businessman was then wired with a listening device and given $ 30,000 to offer Mr Tucker.• He pulled out the next box; there was no mark of any listening device on the wall behind it.• Disguised as toilet equipment all the apparatus was there - the listening device and several sophisticated miniaturized bugs. 