Hint: An alloy is a uniform mixture and is made up of two or more chemical elements out of which at least one element is a metal. Most alloys are prepared by melting the metals, mixing them while they are liquid to form a solution and then leaving them to cool and turn into solid again.
Complete answer: The term alloy means a substance that is formed from the combination of two or more metals. The properties exhibited by alloys are different from the properties of its individual components. Alloys have greater strength and hardness when compared to pure metals. Red gold is an example of an alloy which is produced by alloying copper and gold together. The term intermetallic compound is used to refer to the alloys which have well-defined crystal structures. Brass, bronze and steel are alloys but iron is not an alloy. So, iron is the odd term. Therefore, iron is the odd term.
Note: Alloys are known to feature metallic bonding and for most of the practical applications, the constituents of an alloy are measured in terms of their mass percentages. But, sometimes the components of alloys are measured in terms of their atomic fractions. Note that alloys can be classified into two different categories based on the arrangements of atoms in their respective lattices: substitutional alloys and interstitial alloys. And also remember that alloys can be classified based on the total number of phases present in them. For example, homogeneous alloys have only one phase and heterogeneous alloys have two or more than two phases.