Amplitude modulation was a method of electronic radio communications. This method involved varying the strength of the signal relative to the information being transmitted.
The Iotians of Sigma Iotia II were taught how to make AM radios through materials left by the crew of the Horizon, an Earth starship which visited the planet in 2168. IN 2268, Spock made use of one radio to communicate with the USS Enterprise after he and Leonard McCoy escaped the custody of Bela Okmyx. (TOS: "A Piece of the Action")
Uhura detected AM radio transmissions coming from the planet 892-IV in that same year. (TOS: "Bread and Circuses")