Númenor is the great island kingdom founded by Men in J.R.R. Tolkien’s mythology.
The Valar raised Númenor from the sea as a reward for the Edain who had aided the Elves during the War of Wrath. Elros, the half-elven brother of Elrond, became the first king. Númenóreans received long life, although they remained mortal and were forbidden to sail west to the Undying Lands.
Over many centuries, Númenor became a powerful maritime kingdom. Its rulers eventually grew jealous of the Elves and fearful of death. Under King Ar-Pharazôn, the island’s people challenged the ban and attempted to seize immortality. The island was destroyed in the Downfall of Númenor, and the survivors founded realms in Middle-earth.
Those successor kingdoms were Arnor and Gondor, while Rohan arose later as a separate kingdom. Númenor is therefore not another name for Gondor; it is the lost island civilization from which the great Dúnedain realms descended.