lettre de cachet
noun
let·tre de ca·chet ˈle-trə-də-ˌka-ˈshā 
plural lettres de cachet ˈle-trə(z)-də-ˌka-ˈshā
: a letter bearing an official seal and usually authorizing imprisonment without trial of a named person
Example Sentences
Recent Examples on the Web Sade would remain imprisoned until the Revolution abolished the lettre de cachet in 1790. 
Mitchell Abidor, The New York Review of Books, 12 Feb. 2020
Mitchell Abidor, The New York Review of Books, 12 Feb. 2020 Word History
Etymology
French, literally, letter with a seal
First Known Use
1715, in the meaning defined above