GEANT is the name of a series of simulation software designed to describe the passage of elementary particles through matter, using Monte Carlo methods.
[2] Versions of GEANT through 3.21 were written in FORTRAN and eventually maintained as part of CERNLIB.
Since about 2000, the last FORTRAN release has been essentially in stasis and receives only occasional bug fixes.
Most of GEANT-3 is available under the GNU General Public License,[3] with the exception[4] of some hadronic interaction code contributed by the FLUKA collaboration.
A related (but separate) product is Geant4 (when referring to this version, the name is typically no longer capitalized).