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).
Radio waves are the lowest-frequency waves in the EM spectrum. Radio waves can be used to carry other signals to receivers that subsequently translate these signals into usable information. Many objects, both natural and man-made, emit radio waves. Anything that emits heat emits radiation across the entire spectrum, but in different amounts.