Longman Dictionary of Contemporary English
Wiktionary
n. (context UK journalism English) 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. vb. 1 (context UK English) (non-gloss definition: 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 (context idiomatic UK English) (non-gloss definition: Used to grab attention, implying importance, news-worthiness, etc.)
WordNet
n. late news that is inserted into the newspaper at the last minute
Wikipedia
"Stop Press" or "Stop the Presses" is a phrase stemming from the printed news media industry as an exclamation signifying the discovery of the need to change the content of an issue just before, or during its printing.
Since this meant that the printing press literally had to be stopped or delayed and much of the existing copies of a publication which had already printed had to be discarded - which carried extreme cost, it is a phrase indicating the arrival of extremely significant news or the discovery of an extremely grave error. The phrase is common in an idiomatic context, referring to the discovery of significant information and is often used sarcastically.
Usage examples of "stop press".
Frustrated, impatient, he picked up the newspaper again in an effort to distract himself, and then he saw it, a tiny item on the back page, in the Stop Press.
Street, and there it was in the stop press: the jury had taken only thirteen minutes to find for the defendants.