n. Retrogressive absorption; specifically, a physiological process by which a part or organ, having advanced to a certain state of development, disappears as such by the absorption of its substance into that of a part or organ which replaces it.n. Absorption of some product of the organism, as a tissue, exudate, or secretion.n. In petrography, the melting of a phenocryst in a porphyritic rock and its recrystallization in situ as other minerals. This results in the formation of a resorption border in many cases about the more or less resorbed phenocryst, as when hornblende is surrounded by a black border composed of pyroxene and magnetite.