what is a nickel?
Definition of nickel. 1  1 : a silver-white hard malleable ductile metallic element capable of a high polish and resistant to corrosion that is used chiefly in alloys and as a catalyst â see element table. 2  2a (1) : the U.S. 5-cent piece regularly containing 25 percent nickel and 75 percent copper (2) : the Canadian 5-cent pieceb : five cents.