In simple terms, NDAs are intended to create a confidential agreement between someone who holds a trade secret, and a person to whom the trade secret will be shared.
The legally enforceable contract is intended to protect people's ideas and stop sensitive information about how businesses operate being shared with competitors.
While the origins of how NDAs came into first use is relatively unknown, the first mention of them in newspaper articles appears to date back to the 1940s, relating to maritime law.
They later appeared in the context of new technology firms such as IBM and Microsoft, which, presumably, did not want their trade secrets shared with the rest of the world.
By the 1970s, NDAs started being used to shut down the spread of other types of information. They were signed by consultants working for the US House Select Committee on Assassinations, looking into the killings of Martin Luther King and John F Kennedy, to prohibit them "indicating, divulging or acknowledging" that they even worked on the investigations.
A decade later, the agreements were being used by a wide variety of businesses and started becoming the norm in employment contracts within certain white-collar professions. Crucially, they became a standard part of legal settlement agreements - where employees would agree not to sue a firm or take them to tribunal in exchange for, usually a sum of money.
Fast forward to 2016, when former Fox News anchor Gretchen Carlson tried to use former Fox chairman Roger Ailes, and received a reported $20m to drop the action.
Harvey Weinstein was next to hit the headlines, in 2017, when it emerged he had paid off women alleging a catalogue of sexual offences, ranging from harassment and assault to rape. Weinstein is awaiting trial.
Last year Sir Phillip Green, the owner of retail group Arcadia, is alleged have used NDAs to stop five former staff speaking out about groping, harassment and bullying. He denies the claims that he acted inappropriately and dismissed them as "banter".
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