(1) In chemistry, the force that impels certain atoms or molecules to bind to or unite with certain other atoms or molecules to form complexes or compounds; chemical attraction. (2) Selective staining of a tissue by a dye or the selective uptake of a dye, chemical, or other substance by a tissue. (3) In psychology and psychiatry, a positive bond or relatedness between people or groups, or a person's positive regard for an object, idea, or activity; a positive cathexis. (4) In immunology, the strength of interaction between an antigen binding site and an antigenic determinant. (5) A biomolecular interaction exhibiting specificity. [L. affinis, neighboring, fr. ad, to, + finis, end, boundary]