The Ban of the Valar
the prohibition laid on the Númenóreans against sailing to Aman
Definition
The Ban of the Valar is the prohibition laid by the Valar upon the Men of Númenor, that they were not to sail so far westward that the shores of their own land were lost from sight, and not to set foot upon the land of Aman. It rested on the mortality given to Men as their gift, and on the will that the Númenóreans should not seek to enter by force the guarded realm of the deathless.
History
The Ban was laid at the foundation of Númenor, when the Valar granted the isle to the Fathers of Men, and in the early generations it was kept. As the shadow grew in Númenor, the kings spoke against it: Tar-Atanamir was the first to speak openly against the Valar and against the Ban, and from his days the kings and the people turned from it. The breaking of the Ban under Ar-Pharazôn, who sailed in his Great Armament to assail Aman, brought the wrath that is told in the Akallabêth: the Changing of the World, the Drowning of Númenor, and the rounding of the old earth.
Significance
The Ban is the hinge of the Second Age in Númenor: the keeping or the breaking of it divides the Faithful from the kings of pride, and its breaking undoes the greatest kingdom of Men, binding the story of its Downfall to the memory that the Elendili carried into Middle-earth.