The Free On-line Dictionary of Computing (30 December 2018):

differential backup

    A kind of backup that copies all files
   that have changed since the last full backup.  Each
   differential backup will include all files in previous
   differential backups since the full backup so to restore a
   version of a file, you only need to search the full backup and
   the relevant differential backup.

   Some systems support differential backup by associating an
   "Archive" flag with each file and setting this flag whenever
   the file is modified to indicate that it should be included in
   the next backup.  A differential backup does not change this
   flag, whereas an incremental backup resets it.

   (2004-03-05)