1. See upright. 1. horizontal. 1  being in a position or direction perpendicular to the plane of the horizon; upright; plumb. 2  of, pertaining to, or situated at the vertex. 3  Anatomyof or pertaining to the cranial vertex.
Last Updated: Radio wave, wave from the portion of the electromagnetic spectrum at lower frequencies than microwaves. The wavelengths of radio waves range from thousands of metres to 30 cm. These correspond to frequencies as low as 3 Hz and as high as 1 gigahertz (109 Hz).
Although turning on a radio produces sound, radio waves themselves cannot be heard and have nothing to do with sound waves. While sound waves are a vibration of the air, radio waves are electromagnetic and a part of the light spectrum. Radio waves travel at a speed of 186,282 miles (299,727 kilometers) per secondâthe speed of light.