from
The Free On-line Dictionary of Computing (8 July 2008)
differential backup
<operating system> 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)