Amplitude modulation, or AM, was a method of electronic radio communications used on Earth in the 20th century. (VOY: "The 37's")
The Iotians of Sigma Iotia II learned how to build AM radios with textbooks left behind by the crew of the Horizon, a starship that visited the planet in 2168, before the prime directive was adopted by Starfleet.
In 2268, Spock made use of an Iotian AM 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 and FM radio transmissions coming from the planet 892-IV in that same year. (TOS: "Bread and Circuses")
In 2371, the crew of USS Voyager picked up an SOS on the AM radio of a 1936 Ford found adrift in space in the Delta Quadrant. (VOY: "The 37's")
In 2376, the people of Kelemane's planet invented AM radio and attempted to contact the Voyager. (VOY: "Blink of an Eye")