An orgy (όργιον) was a secret cultic congregation at nighttime in Ancient Greek religion, overseen by an orgiophant (a teacher or revealer of secret rites). In modern parlance an orgy may refer to an event involving group sex.
Greek όργιον is cognate with έργον "work, effect, [religious] service" (ultimately cognate to English work itself). The term was lent into English in the 16th century, via the Latinized orgia.