Etymology
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affinity (n.)

c. 1300, "relation by marriage" (as opposed to consanguinity), from Old French afinite "relationship, kinship; neighborhood, vicinity" (12c., Modern French affinité), from Latin affinitatem (nominative affinitas) "relationship by marriage; neighborhood," noun of state from affinis "adjoining, adjacent," also "kin by marriage," literally "bordering on," from ad "to" (see ad-) + finis "a border, a boundary" (see finish (v.)).

Spelling was re-Latinized in early Modern English. Used figuratively in English since c. 1600 of structural relationships in chemistry, philology, geometry, etc. Meaning "natural liking or attraction, a relationship as close as family between persons not related by blood" is from 1610s.

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Definitions of affinity

affinity (n.)
(immunology) the attraction between an antigen and an antibody;
affinity (n.)
(anthropology) kinship by marriage or adoption; not a blood relationship;
affinity (n.)
(biology) state of relationship between organisms or groups of organisms resulting in resemblance in structure or structural parts;
in anatomical structure prehistoric man shows close affinity with modern humans
Synonyms: phylogenetic relation
affinity (n.)
a close connection marked by community of interests or similarity in nature or character;
found a natural affinity with the immigrants
Synonyms: kinship
affinity (n.)
the force attracting atoms to each other and binding them together in a molecule;
basic dyes have an affinity for wool and silk
Synonyms: chemical attraction
affinity (n.)
inherent resemblance between persons or things;
affinity (n.)
a natural attraction or feeling of kinship;
the mysterious affinity between them
James's affinity with Sam
an affinity for politics
From wordnet.princeton.edu