Noun 1. a substance composed of two or more metals, or of a metal or metals with a nonmetal, intimately mixed, as by fusion or electrodeposition. 2. a less costly metal mixed with a more valuable one. 3. standard; quality; fineness. 4. admixture, as of good with evil. 5. anything added that serves to reduce quality or purity. verb 1. to impair or debase by admixture 2. to reduce the purity of by mixing with a less valuable metal 3. to mix so as to form an alloy