Source : Free On-Line Dictionary of Computing

incremental backup
     
         A copy of all files which have changed
        since the date of some previous backup.  The first backup of a
        file system should include all files - a "{full backup}".
        Call this level 0.  The next backup could also be a full level
        0 backup but it is usually much quicker to do a level 1 backup
        which will include only those files which have changed since
        the level 0 backup.  Together the level 0 and level 1 backups
        will include the latest version of every file.  Level 1
        backups can be made until, say, the backup tape is nearly
        full, after which we can switch to level 2.  Each level
        includes those files which have changed since the last backup
        at a lower level.  The more levels you use, the longer it will
        take to restore the latest version of a file (or all files) if
        you don't know when it was last modified.
     
        (2003-06-22)