An alibi in the general sense is information that attempts to show that a person was not at the place where an act they are being accused of was committed. In Latin alibi literally means "somewhere else".
In the Duke lacrosse case, two of the three defendants had alibis. One of them, Reade Seligmann, revealed his alibi to the public within 48 hours of his arrest. Shortly after, Seligmann's alibi witness was arrested on a years-old warrant in an unrelated case; he said that the detective who arrested him had asked first if he had "anything new" to say about the lacrosse case and then arrested him when he said no. Nifong also changed the timeline of his theory of the case shortly thereafter. Colin Finnerty revealed publicly during the case that he had alibi evidence but he did not reveal what that evidence was until Attorney General Roy Cooper declared the players innocent.