A Nixie is a name given by the United States Postal Service to a piece of mail which is undeliverable as addressed. It is derived from "nix", English slang for the German nichts ("nothing"), and "-ie", an item or a thing. The term is thought to date from the late 19th century.
The term "Nixie clerk" originally referred to a postal employee who tried to figure out what to do with undeliverable items, which were not just poorly addressed mail, but ranged from torn-open envelopes of photographs, even to lost and found wallets dropped into a mailbox. More recently, the USPS National Change of Address (NCOA) Service provided data to mailers with scoring on how close a match the name and address are to something actually forward-able or deliverable, which were referred to as "Nixie Codes".
The USPS distinguishes a Nixie from other address errors in that the mail piece is always returned to the sender, whereas a change of address could either be forwarded or returned to the sender with a correction or notification. In the early 21st century it began to be printed in the upper left corner of yellow labels generated by the USPS's Postal Automated Redirection System (PARS).