The term retcon is a term derived form of the phrase "retroactive continuity". It originated decades ago in the comic-book-collecting community, though its usefulness has proven itself far beyond that singular medium. Now, it refers to any plot development or revelation that introduces new information about previously-established events, especially if the new information had not been an intended element of the original history and or contradicts established continuity events.
Being slang, there is no strict definition of what constitutes a retcon; the most liberal usage applies the term to any new backstory introduced with sufficient distance from the original material, be it in terms of time or authorship or both. However, a stricter usage would confine it to developments that change or contradict past events. Thus, for some, "retcon" carries a negative overtone, no doubt due in part to the fact that the term is naturally brought up when people are talking about continuity problems.