A legal prerogative asserted by the French monarchy by which, at his discretion, the French King answered to no-one and could order whatever he wished.
The kings began to hand these out indiscriminately to courtiers or religious leaders in exchange for favors to the Crown. They were even signed and given out in blank.
Initially, these orders were given verbally but later, the customary law was changed to require that the order be in written and signed by the king or bearing his seal; hence lettre de cachet.
In France, the word for hidden is caché. Thus, it was implicit in these orders that the monarch's deliberation process, if any, prior to issuing these orders, was secret and hidden from the interested parties.
Over the years, the lettres de cachet were used to incarcerate and hold indefinitely - or exile - any person who the king felt was a threat to his person.
Many of the first settlers of the present state of Louisiana arrived because of their forced exile as set out in a lettre de cachet.
Such drastic legal rights and the inevitable abuses it led to, contributed to the French Revolution and the dramatic end of the French monarchy.
Today, the term is often used to derogate a document which purports to affect a person's rights without providing the person so affected a full hearing as to the tenure of the document. For example, evidence submitted in camera on a national security matter or the involuntary institutionalization of a person under mental health legislation.
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