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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]
raft1 a flat platform made of buoyant materials such as wood or air-filled barrels, designed to float and often to support or carry people or goods. [6 definitions]
raft2 (informal) very many; a large quantity.
rafter one of the usu. sloping timbers that support the covering material of a roof.
rag1 a scrap or fragment of cloth. [5 definitions]