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.