In the broad sense, the conservation of a document with no time limit. In the sense of public archives, documents which are archived, after the limit of their administrative use, are by definition long term. See also the distinction between long-term preservation and secured backups on the website of the cines: https://www.cines.fr/archivage/un-concept-des-problematiques/le-concept-darchivage-numerique-perenne/