Radio is the transmission of information and reception of information, using electromagnetic waves. Some devices used for radio communications are also known simply as a "radio."
The term radio is also sometimes used in referenced to more advanced communications technologies, the word having become synonymous with near instantaneous audio communications. (TOS comic: "Captain's Pleasure")
The Cape Canaveral radio activity of 1968 was instrumental in the success of Kirk and crew's obtaining sensitive data on this time period for 23rd century historians. (TOS episode: "Assignment: Earth")
The Enterprise accidentally hailed the U.S.A.F., in 1969, believing it to be Federation Starfleet. Transcripts of this communique'-not destroyed-was later decoded and became the reference Star Fleet Technical Manual. (TOS episode: "Tomorrow is Yesterday")
Radiation and scattered radio reception of 1986 San Francisco Bay made Chekov unable for beam up by transporter, albeit a cloaked, underpowered Bird of Prey locking on to his communicator. (TOS movie: Star Trek IV: The Voyage Home)
In 2254, radio bursts from the ill-fated SS Columbia distress call diverted the USS Enterprise to the Talos star group. In reality, these radio beams were fabricated by the combined telepathic strength of the Talosians. (TOS episode: "The Cage")
In 2268, Spock hailed the USS Enterprise from a radio station on Sigma Iotia II. Uhura was able to reply to the unusual frequency, as both the science officer and Doctor McCoy had their communicators confiscated. (TOS episode: "A Piece of the Action")
In 2268, Isak mistakenly believed that Kirk and Spock were attempting to build a radio while imprisoned on Ekos. The orbital missile attack on Zeon was to be orchestrated by radio command. (TOS episode: "Patterns of Force")