"The Label Maker": Season 6, Episode 12
This episode changed the entire economy of gift giving. Jerry gives his Super Bowl tickets to Tim Whatley, who in return gives Jerry the titular label maker. Elaine happened to give Whatley the same type of label maker, leading her to believe that the dentist "regifted" the item. Jerry, meanwhile, wants to "degift" his Super Bowl tickets and ends up regifting the label maker to the roommate of George's new girlfriend. Long story short, this episode popularized this term that describes what we do with the presents we can't return for store credit.