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).
Though radio waves are often used to encode and transmit data that is eventually transformed into sound (similar to how sound is encoded and transmitted electrically), they are not sound waves themselves. Radio waves are electromagnetic waves and so they travel at the speed that all electromagnetic waves travelâ the speed of light.