A radio wave is a wave on the electromagnetic spectrum. This wave is used for radio frequencies.
In 1996, Thomas Paris theorized that radio wave readouts were from radio telescope transmissions. (VOY: "Future's End")
In the early 23rd century, radio-interference distress calls were transmitted on a radio wave. (TOS: "The Cage", "The Menagerie, Part I")
In 2267, Lieutenant Nyota Uhura monitored the radio waves on planet 892-IV. She referred to these waves as being of the "old style". (TOS: "Bread and Circuses")
In 2364, Lieutenant Geordi La Forge informed Doctor Beverly Crusher that he could see radio waves in his VISOR. (TNG: "Encounter at Farpoint")