Database file used by IBM Lotus business software; saved in the Notes Storage Facility (NSF) format, which stores data in document units called "notes;" used for storing collaborative business information such as mail and calendar data.

NSF files are often hosted by a Lotus Domino server, and they can be downloaded by Lotus Notes clients in order to work with a fully functional database in offline mode. If the client makes changes to the database, Domino includes the facilities to synchronize the changes back to the master database. NSF files can also be used for the process of data replication. NOTE: NSF files may also serve as a Domino Web application that is hosted on a Web server, where the URL has a trailing ".nsf" string. These files include the Domino Web Service Design Element, which enables Web access.