A pejorative name for some versions of the
GNU project
copyleft or
General Public License (GPL),
which requires that any tools or application programs
incorporating copylefted code must be source-distributed on
the same terms as GNU code. Thus it is alleged that the
copyleft "infects" software generated with GNU tools, which
may in turn infect other software that reuses any of its code.
Copyright law limits the scope of the GPL to "programs
textually incorporating significant amounts of GNU code" so
GPL is only passed on if actual GNU source is transmitted.
This used to be the case with the
Bison parser skeleton
until its licence was fixed.
http://org.gnu.de/manual/bison/html_chapter/bison_2.html#SEC2.