"pertaining to the radiometer or to experiments performed by it," 1877, from radiometer "instrument to transform radiant energy into mechanical work" (1875), radiometry, from radio-, here indicating "radiant energy," + -metric. Previously radiometer was the name of an old cross-staff instrument for measuring angles. Radiometric dating is attested from 1906.

"examination by means of x-rays," 1896, from radio- + -scopy.

"telephone link through which the signal is transmitted partly by radio," 1900, from radio (n.) + telephone (n.).

"treatment of disease by means of x-rays," 1902, from radio- + therapy.

cruciferous plant cultivated from antiquity for its crisp, slightly pungent, edible root, Middle English radich, from late Old English rædic "radish," from Latin radicem (nominative radix) "root, radish" (from PIE root *wrād- "branch, root"). The spelling in English is perhaps influenced by Old French radise, variant of radice, from Vulgar Latin *radicina, from radicem.

radioactive metallic alkaline earth element, 1899, from French radium, formed in Modern Latin from Latin radius "ray" (see radius). With metallic element ending -ium. Named 1898 after identification by Marie Curie and her husband; so called for its power of emitting energy in the form of rays.

1590s, "cross-shaft, straight rod or bar," from Latin radius "staff, stake, rod; spoke of a wheel; ray of light, beam of light; radius of a circle," a word of unknown origin. Perhaps related to radix "root," but de Vaan finds that "unlikely." The classical plural is radii. 

The geometric sense of "straight line drawn from the center of a circle to the circumference" is recorded from 1650s. Meaning "circular area of defined distance around some place" is attested from 1853. As the name of the shorter of the two bones of the forearm from 1610s in English (the Latin word had been used thus by the Romans).

the heaviest gaseous element, short-lived and radioactive, 1918, from German Radon, from radium (q.v.) + -on suffix of inert gases. The element was identified in radioactive decay of radium. Alternative name niton (from Latin nitens "shining") gained currency in France and Germany.

1753, a type of surgical instrument, from Latin radula "scraper, scraping iron," from radere "to scrape" (see raze (v.).  As "tongue or lingual ribbon of a mollusk," by 1853. Related: Radular.