Osiris (from ancient Greek Ὄσιρις) is a god of the Egyptian pantheon and a mythical king of ancient Egypt. Inventor of agriculture and religion, his reign was beneficent and civilizing. He died drowned in the Nile, assassinated in a plot organised by Set, his younger brother. Despite the dismemberment of his body, he is revived by the magical power of his sister Isis. The martyrdom of Osiris earned him the right to enter the world beyond, where he became the sovereign and supreme judge of the laws of Ma'at.

In the Middle Kingdom, the city of Abydos became the city of the god Osiris. It thus attracted many worshippers in search of eternity. The city's fame rests on its New Year's Eve festivities and on a holy relic, the head of the god.

During the first millennium BC, Osiris retained his status as a funerary god and judge of souls. However, his aspects as god of the Nile's waters and, by the same token, as god of fertility, acquired primacy, thus increasing his popularity among the Nilotic population. Greek settlers in Memphis adopted his cult as early as the fourth century BC in its local form of Osiris-Apis, the dead and mummified sacred bull. The Lagid rulers imported this cult into their capital Alexandria in the form of Serapis, the syncretic Greek-Egyptian god. After the conquest of Egypt by the Roman forces, Osiris and Isis were exported to Rome and its empire. They remained there, with ups and downs, until the fourth century AD, when they were finally ousted by Christianity (prohibition of paganism following the Edict of Thessalonica). The Osirian cult, active since the xxth century BC, lasted until the life of our era, when the temple of Isis on the island of Philæ, the last one in Egypt, was closed around 530, a closure ordered by the emperor Justinian.

The theonym Osiris is a transliteration into the Latin alphabet of a word from ancient Greek: Ὄσιρις, which itself originates from a word in the Egyptian language: Wsjr, variably transliterated according to authors as Asar, Asari, Aser, Ausar, Ausir, Wesir, Ousir, Ousire or Ausare, the original Egyptian pronunciation not being known due to the fact that the Egyptian hieroglyphic script does not reproduce all vowels. Several Egyptologists have tried to give a meaning to the theonym Osiris. In 1980, John Gwyn Griffiths proposed that Wsjr derives from Wser and means 'the Mighty One'. Moreover, one of the oldest attestations of the god Osiris appears in the mastaba of the deceased Netjer-ouser (God-powerful). In 1987, Wolfhart Westendorf proposed the etymology Waset-jret: the one who carries the eye. In 1985, David Lorton hypothesised that Wsjr is a compound word derived from the morpheme set associated with jret; set-jret meaning 'ritual activity'. Osiris would then be the one who benefits from ritual activity. According to the Egyptian vision, the destructive forces are in perpetual struggle against the positive forces. In this, Set is opposed to his brother Osiris, symbol of the fertile and nourishing earth.