What is alloy? What is alloy? A homogeneous mixture or solid solution of two or more metals, the atoms of one replacing or occupying interstitial positions between the atoms of the other: Brass is an alloy of zinc and copper. A mixture; an amalgam: "Television news has . . . always been an alloy of journalism and show business” ( Bill Moyers). The relative degree of mixture with a base metal; fineness.Something added that lowers value or purity.To combine (metals) to form an alloy.To combine; mix: idealism that was alloyed with political skill. To debase by the addition of an inferior element.