affinity [uh-fin-i-tee]  noun  1  a natural liking for or attraction to a person, thing, idea, etc.  2  a person, thing, idea, etc., for which such a natural liking or attraction is felt.  3  a relationship by marriage or by ties other than those of blood.  4  inherent likeness or agreement; close resemblance or connection.  5  Biology the phylogenetic relationship between two organisms or groups of organisms resulting in a resemblance in general plan or structure, or in the essential structural parts. 6  Chemistry the force by which atoms are held together in chemical compounds.