A retcon is the alteration of previously established facts in the continuity of a fictional work.
Retcon motivations Edit
There are various motivations for retconning. The changes may occur to accommodate sequels or derivative works, allowing newer authors or creators to revise the history to include a course of events that would not have been possible in the story's original continuity. Retcons allow for authors to reintroduce popular characters and resolve errors in chronology.
See also: The Time War
In Doctor Who, the drug B67 is popularly known as "retcon". It can erase recent memories, or even replace them with false memories. As Torchwood 3 discovered, this drug has many uses. Not just the one obvious use; e.g., Gwen also used ti to allow her to confess her affair to her husband so she could stop feeling guilty, and then immediately make him forget it so she wouldn't get in trouble.
But before you judge Torchwood, remember that Superman has the same power, only he doesn't use a pill, he uses sexualized physical contact like kissing.