(1) The transmission of wireless signals (electromagnetic waves) over the air or through a hollow tube called a "waveguide." Although "radio" is often thought of as only AM and FM or sometimes two-way radio, all transmission systems that propagate signals through the air are some form of "radio," including TV, satellite, portable phones, cellphones and wireless LANs. See spectrum.
The radio portion of the entire spectrum of radiation is from 3 kHz to 300 GHz. This huge band of frequencies has been defined by the FCC in the U.S. and governmental bodies in other countries.
(2) An electronic circuit that transmits and receives wireless signals (electromagnetic waves). The phrase "the device has four radios" means the unit has some combination of receivers and transceivers, which may reside on one or more chips. See transceiver.
A Four-Radio Chip From TI
In addition to a cellular radio, modern smartphones have other radios. This single chip has four: two receivers (FM and GPS) and two transceivers (Wi-Fi and Bluetooth). (Image courtesy of Texas Instruments, Inc.)
(3) A device that receives wireless audio signals (electromagnetic waves) such as an AM/FM radio or satellite radio. See Vintage Radio Museum.
For the Early Adopter!
In 1925, listeners tuned three stages on this battery-operated Stewart Warner Model 325 to get a good signal. Once they got one, they logged the numbers so they could tune in quickly the next time. (Equipment courtesy of Vintage Radio and Communications Museum of Connecticut, www.vrcmct.org)
A Double Thrill for Dad
Not only was it a pleasure for John Coolidge to have his son Calvin accept the Republican nomination for U.S. president in Cleveland, Ohio in 1924, but to actually hear the speech was quite exciting. (Image courtesy of Vintage Radio and Communications Museum of Connecticut, www.vrcmct.org)