Secret rite in the cults of ancient Greek or Roman deities involving singing and dancing and drinking and sexual activity WordNet 3.0 & Open English WordNet

In modern usage, an orgy is a sex party where guests freely engage in open and unrestrained sexual activity or group sex. Wikipedia

A secret rite in the cults of ancient Greek or Roman deities involving singing and dancing and drinking and sexual activity. OmegaWiki

Originally, secret rites or ceremonies, typically involving riotous and dissolute behaviour, including dancing, drunkenness and indiscriminate sexual activity, undertaken in honour of various pagan gods or goddesses (such as Attis, Bacchus, Ceres, Dionysus, Osiris, etc). Wiktionary