sig block /sig blok/ n. [Internet and Usenet; often written `.sig'
   there] Short for `signature', used specifically to refer to the
   electronic signature block that most Unix mail- and news-posting
   software will {automagically} append to outgoing mail and news. The
   composition of one's sig can be quite an art form, including an ASCII
   logo, one's choice of witty sayings (see {sig quote}, {fool file}), or
   even source code for small programs about which the author wishes to
   make a statement; but many consider large sigs a waste of {bandwidth},
   and it has been observed that the size of one's sig block is usually
   inversely proportional to one's longevity and level of prestige on the
   net. See also {doubled sig}, {McQuary limit}.


Source: The Jargon File