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).
"having rays, furnished with rays or ray-like parts, shining," 1660s, from Latin radiatus, past participle of radiare "to beam, shine, gleam; make beaming," from radius "beam of light; spoke of a wheel" (see radius).
"one of the Radiolaria," a name applied by Haeckel (1862) to the protozoa called by Ehrenberg Polycystina. The classification name is Modern Latin, from Latin radiolus, diminutive of radius (q.v.), so called for the organisms' radiant "spikes."
1898, of an atomic nucleus, "capable of spontaneous nuclear decay releasing ionizing emissions," from French radio-actif, coined by Pierre and Marie Curie from radio-, combining form of Latin radius "ray" (see radius) + actif "active" (see active). Of processes, etc., "involving or produced by radioactivity," by 1903.
1610s, "issue or spread in all directions from a point in rays or straight lines," from Latin radiatus, past participle of radiare "to beam, shine, gleam; make beaming," from radius "beam of light; spoke of a wheel" (see radius). Meaning "be radiant, give off rays (of light or heat)" is from 1640s. Related: Radiated; radiates; radiating.
mid-15c., radiacion, "act or process of emitting light," from Latin radiationem (nominative radiatio) "a shining, radiation," noun of action from past-participle stem of radiare "to beam, shine, gleam; make beaming," from radius "beam of light; spoke of a wheel" (see radius).
Meaning "rays or beams emitted" is from 1560s. Meaning "divergence from a center" is 1650s. In modern physics, "emission or transmission of energy in the form of waves or particles," especially in reference to ionizing radiation, from early 20c.
c. 1400, "of or like a ray or radius," from Medieval Latin radialis, from Latin radius "shaft, rod; spoke of a wheel; beam of light" (see radius). Meaning "arranged like the radii of a circle" is by 1750. As a noun, "a radiating or radial part," by 1872. As a type of tire, attested from 1965, short for radial-ply (tire), so called because the cords run at right angles to the circumference. Related: Radially.