Tycho Magnetic Anomaly Two, or TMA-2, was a large, black, crystalline monolith found in the Lagrange point between Io and Jupiter, discovered by David Bowman upon the arrival of Discovery in orbit around Jupiter.
TMA-2 was created by the Firstborn, who refered to it as the "Stargate." The reason for this name was that the Firstborn used the monolith not only to continue their experiments in our solar system, but also to create a sort of wormhole which could move any nearby and desired object anywhere in the galaxy.
TMA-2 is a bit of a misnomer (as pointed out by Dave Bowman) as TMA-2 was found millions of miles from the moon, let alone Tycho, nor did it produce a significant magnetic field. TMA-2 was often referred to by Heywood Floyd as "Big Brother", in reference to Dave Bowman, who described the monolith as TMA-1's "big brother" when he first encountered it. The Russian crew of the Cosmonaut Alexei Leonov referred to the Jupiter monolith as Zagadka, the Russian word for "enigma".
In the novel of 2001: A Space Odyssey, TMA-2 was found, not in orbit around Jupiter, but standing erect on the surface of Iapetus, one of Saturn's many moons. The reason for this is that the climax of the story was set at Saturn in the novel; in the film version, the climax was set at Jupiter, as the special effects specialist could not produce a convincing model of Saturn's rings. However, in Clarke's sequel to 2001, 2010: Odyssey Two, the events were placed at Jupiter, rather than Saturn. Therefore, from hereafter, TMA-2 will indeed be called the "Jupiter Monolith".