Autoconf exception
A GPL exception that lets configure scripts generated by GNU Autoconf be distributed under any licence.
Autoconf is GPL software, and the script it emits contains large chunks of Autoconf's own code. Without an exception, every project shipping a configure script – proprietary ones included – would arguably have to GPL it. Nobody wanted that outcome, least of all the FSF.
The exception states that the generated output may be distributed under whatever terms the project chooses. With GPLv3 in 2009 it gained the formal name Autoconf Configure Script Exception, version 3.0. It is a sibling of the Bison exception and GCC's runtime exception: the tool stays copyleft, the things it produces do not.