A radio scanner or simply scanner is a radio receiver that can
automatically tune discrete frequencies, scanning over a frequency band to
find a signal until the initial transmission ceases.
The term scanner generally refers to a communications receiver that is
primarily intended for monitoring VHF and UHF landmobile radio systems, as
opposed to, for instance, a receiver used to monitor international shortwave
transmissions, although these may be classified as scanners too.
More often than not, these scanners can also tune to different types of
modulation as well (AM, FM, SSB, etc.). Early scanners were slow, bulky, and
expensive.