Noun

1. The event or news article important enough to delay or interrupt the print, or require a reprint, of a publication, particularly of a newspaper edition.

Interjection

1. Used to announce an event or news article important enough to delay or interrupt the print, or require a reprint, of publication, particularly of a newspaper edition.
2. Used to grab attention, implying importance, news-worthiness, etc.

Quotations

• 1989, Textual Introduction to The Anatomy of Melancholy, Robert Burton http://books.google.com/books?ie=UTF-8&hl=en&id=C64iQVqCm3MC&pg=RA1-PR51&lpg=RA1-PR51&sig=JpcrTaGG5EdPxJjr2U8VwuilsEw

  Three of the errata corrections had already been made as stop-press corrections [...]

• 2002, Russell Miller, Behind the Lines http://books.google.com/books?ie=UTF-8&hl=en&id=iAtuxaBlfRkC&pg=PA103&lpg=PA103&sig=erWM7_pq0H8pCDJnv5W4-5bxlv0

  Faked Stop Press! announcements in newspapers. A valuable trick because it can always be claimed that the announcement was ‘hushed up’. It is also easy to fake the printing of the Stop Press.

• 2005, Mary Norway, The Sinn Fein Rebellion As I Saw It, http://books.google.com/books?ie=UTF-8&hl=en&id=kdp6DLzb6KcC&pg=PA65&lpg=PA65&sig=TWkyRKxWPtmbwiGPh_BvuIQNwlA

  Another lady thought she would drive a lesson home, so she said: “But you said it was a ‘Stop press,’ and you knew it was not.”

  “It is, miss, but sure they hadn’t time to print the ‘stop press’ on it!!”

  (“Stop press” is the latest news, usually printed on the back of the paper.)