radiopaque dye(n = noun.substance) - dye that does not allow the passage of X rays or other radiation; used to outline certain organs during X-ray examination;
is a kind of dye, dyestuff

acid dye, aniline dye, azo dye, basic dye, cyanine dye, direct dye, double dye, dye, fast dye, fluorescent dye, hair dye, lac dye, metallized dye, radiopaque, substantive dye, vat dye