"Retcon" (short for "retro-active continuity") is a term used in fiction for a piece of information that is included as a way to address an apparent continuity error or inconsistency. Retcons are common-place in long-running fictional universes (such as comic books and popular series of novels and television shows).
Retcons are not necessarily the same as information that had previously been unrevealed. For example, the revelation that the criminal Mumbles had survived his first encounter with Dick Tracy and had not been lost at sea was not a retcon. However, when writer Max Allan Collins revealed that Mumbles had also survived his SECOND encounter with Tracy (when the criminal had specifically been stated to be deceased), that would qualify as a retcon.
Retcons can sometimes be used to address continuity errors, though there are several instances throughout the history of the Dick Tracy comic strip where continuity cannot easily be reconciled.