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).
Tech Talk: Radio Waves in Astronomy. Radio waves are electromagnetic waves (light) with wavelengths between 1 millimeter (one thousandth of a meter) and 100 kilometers (one kilometer is equal to one thousand meters). In terms of frequency, this is equivalent to 300 Gigahertz (one Gigahertz is equal to one billion Hertz) and 3 kilohertz.