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. 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.