The cross-vendor C++ Application Binary Interface. A namespace alias to __cxxabiv1, but user programs should use the alias 'abi'.
A brief overview of an ABI is given in the libstdc++ FAQ, question 5.8 (you may have a copy of the FAQ locally, or you can view the online version at http://gcc.gnu.org/onlinedocs/libstdc++/faq/index.html#5_8).
GCC subscribes to a cross-vendor ABI for C++, sometimes called the IA64 ABI because it happens to be the native ABI for that platform. It is summarized at http://www.codesourcery.com/cxx-abi/ along with the current specification.
For users of GCC greater than or equal to 3.x, entry points are available in <cxxabi.h>, which notes, 'It is not normally necessary for user programs to include this header, or use the entry points directly. However, this header is available should that be needed.'