Secondary database file used by Microsoft SQL Server; created when the primary .MDF database becomes too large (such as when there is no disk space left); may also be manually added to the SQL Server File Group to distribute data between the two files.

NDF files are typically stored on a different hard disk or partition than the primary database file. Each NDF file uses the same filename as the corresponding MDF file, such as "mydb.mdf" and "mydb.ndf."