Meaning

noun (immunology) the attraction between an antigen and an antibody

noun (anthropology) kinship by marriage or adoption; not a blood relationship

noun (biology) state of relationship between organisms or groups of organisms resulting in resemblance in structure or structural parts

originFrom Middle English affinite, from Old French affinité. Ostensibly equivalent to affine + -ity. See also af-.

Etymology from Wiktionary, CC BY-SA 3.0