(1) See
digital signature and
email signature.
(2) A pattern used for matching. Also called a "fingerprint" or "definition." For example, antivirus companies maintain a database that contains the virus code (the "binary signature") of each of the known viruses. To detect a virus, the antivirus program looks for these code strings in executable programs. Spyware blockers that look for spyware and adware also use signature patterns.
An intrusion detection system also uses signatures, which are patterns that suggest an attack. For example, excessive logins that failed or the execution of certain programs.
(3) A unique number built into hardware or software for identification.
(4) A group of printed pages used in the construction of a book or booklet. Typically comprising 16 or 32 pages, signatures may also be 8, 12, 24, 48 or 64 pages long. The signature is printed on one large sheet of paper in a certain "imposition" order that, when cut and folded, results in the correct page sequence. The signatures are then bound together to make the final product.