“Retcon is retroactive continuity–the science of making stuff fit and getting better stories out of it.”―Karen Traviss[src]
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 previous material. However, the current series of Star Wars prequels do qualify as "new source material", and many fans have pointed out instances which apparently retcon elements of the original trilogy.