A kind of backup that copies all files which have changed since the date of the 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.
Compare differential backup.