Retroactive continuity—commonly contracted to the portmanteau word retcon—refers to deliberately changing previously established facts in a work of serial fiction. The change itself is referred to as a retcon, and the act of writing and publishing a retcon is called “retconning.”
When George Lucas re-edited the original Star Wars trilogy, he made changes directly to the source material, rather than introduce new source material that contradicted the contents of the previous material. However, the Star Wars prequels qualified as “new source material,” and many fans have pointed out instances which apparently retcon elements of the original trilogy.
Although the concept is older, the term “retroactive continuity” was popularized in 1983 by DC Comics writer Roy Thomas, and the shortened form “retcon” on Usenet in the late ’80s.