In ancient Greek religion and mythology, the twelve Olympians are the major deities of the Greek pantheon, commonly considered to be Zeus, Hera, Poseidon, Demeter, Athena, Apollo, Artemis, Ares, Aphrodite, Hephaestus, Hermes, and either Hestia or Dionysus.[2] They were called Olympians because, according to tradition, they resided on Mount Olympus.
Although Hades was a major ancient Greek god, and was the brother of the first generation of Olympians (Zeus, Poseidon, Hera, Demeter, and Hestia), his realm was the underworld, far from Olympus, and thus was not usually considered to be one of the Olympians.
Olympus - The mountain of Gods 