A Nixie is an undeliverable-as-addressed mailing.
As used by the United States Postal Service, a Nixie is distinguished from other address errors in that the mail piece is always returned to the sender. (Vs. a Change of Address (COA) which can either be forwarded or returned to the sender corrected/notified.) The term itself has been used to describe undeliverable mail for some time (see below), but starting in the early 21st century it has appeared in print, visible to patrons, in the upper left corner of yellow labels generated by the USPS's Postal Automated Redirection System (PARS).
At some point in the 20th century, the term "Nixie clerk" 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 & 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".
With the advent of e-mail messages, the term receives wider use, e.g. address changes are registered in what may be called the Nixielist (Nixie-list or Nixie list).
Etymology: From "nix", English slang for German variant of "nichts", meaning "nothing"; + "-ie", an item or a thing. "Nix" usage in English c. 1780-1790, "Nixie" c. 1880-1885.