Original Sin:
Original sin may be taken to mean: (1) the sin that Adam committed; (2) a consequence of this first sin, the hereditary stain with which we are born on account of our origin or descent from Adam.
From the earliest times the latter sense of the word was more common, as may be seen by St. Augustine's statement: "the deliberate sin of the first man is the cause of original sin" (De nupt. et concup., II, xxvi, 43). It is the hereditary stain that is dealt with here. As to the sin of Adam we have not to examine the circumstances in which it was committed nor make the exegesis of the third chapter of Genesis.
Also: Used with the definite article ("the original sin"), it refers to the first sin, committed when Adam and Eve succumbed to Satan's temptation.