Serapis was a Greco-Egyptian deity representing both abundance and resurrection.
The cult of Serapis was introduced during the 3rd c. BC on the orders of Pharaoh Ptolemy I Soter in Ptolemaic Egypt as a means to unify the Greeks and Egyptians in his realm.
A serapeum was any temple or religious precinct devoted to Serapis. The cult of Serapis was spread as a matter of deliberate policy by the Ptolemaic kings, who also built the immense Serapeum of Alexandria.
Serapis was depicted as Greek in appearance but with Egyptian trappings, and combined iconography from a great many cults, signifying both abundance and resurrection. Though Ptolemy I may have created the official cult of Serapis and endorsed him as a patron of the Ptolemaic dynasty and Alexandria, Serapis was a god derived from the worship of the Egyptian gods, Osiris and Apis, and also gained attributes from other deities, such as chthonic powers linked to the Greek deities, Hades and Demeter, and benevolence linked to Dionysus.
Serapis continued to increase in popularity during the Roman Empire, replacing Osiris as the consort of Isis in temples outside of Egypt.
At Rome, Serapis was worshiped in the Iseum Campense, the sanctuary of Isis built during the Second Triumvirate in the Campus Martius. The Roman cults of Isis and Serapis gained popularity in the 1st c. AD when Vespasian became emperor. From the Flavian Dynasty on, Serapis was one of the deities who might appear on imperial coinage with the reigning emperor.