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radio spectrum the portion of the electromagnetic spectrum consisting of radio waves, esp. those most frequently used in radio and television broadcasting, from ten kilohertz to 300 gigahertz.
radio station an organization equipped for broadcasting radio. [2 definitions]
radiotelegraph a telegraph in which signals are transmitted by radio rather than by wires.
radiotelegraphy the design, construction, or operation of telegraphs whose signals are transmitted by radio waves.
radiotelephone a telephone whose sound is transmitted by radio waves; radiophone.
radio telescope a concave reflector or array of reflectors designed to collect and focus extraterrestrial radio waves for analysis.
radiotherapy the treatment of an illness with radioactive substances or x-rays.
radio wave an electromagnetic wave with a wavelength of between one millimeter and thirty thousand meters, or a frequency of between ten kilohertz and three hundred thousand megahertz.
radish one of a group of herbs of the cabbage family that have a pungently flavored, crisp, edible root. [2 definitions]
radium a radioactive chemical element that has eighty-eight protons in each nucleus and that occurs in pure form as a solid metal, used esp. as a radiation source for cancer treatment. (symbol: Ra)
radium therapy the treatment of disease, esp. cancer, by means of radium.
radius a straight line segment extending from the center to the surface of a sphere or from the center to the circumference of a circle. [7 definitions]
radius vector a straight line that joins the origin of a vector, at the intersection of two coordinates, to a given point on the same plane. [2 definitions]
radome a dome-shaped structure that protects a radar antenna from the weather, esp. on aircraft, without affecting reception.
radon a radioactive chemical element of the inert gas group that has eighty-six protons in each nucleus and that is produced by the radioactive decay of trace amounts of radium in the soil. (symbol: Rn)
radula in mollusks, a ribbonlike structure in the mouth that bears many rows of teeth.
RAF abbreviation of "Royal Air Force."
raffia a type of palm native to Madagascar, or the fiber harvested from its leafstalks, used for rope, basketweaving, and the like. [2 definitions]
raffinose a white crystalline sugar obtained from the sugar beet, cottonseed, and the like.
raffish carelessly unconventional or disreputable, sometimes appealingly so. [2 definitions]
raffle a type of lottery in which participants buy tickets, one of which is drawn to determine the prizewinner. [2 definitions]