Henosis (Ancient Greek: ἕνωσις) is the classical Greek word for mystical "oneness", "union" or "unity". In Platonism, and especially Neoplatonism, henosis is unification with what is fundamental in reality: the One (Τὸ Ἕν), the Source, or Monad. The Neoplatonic concept has precedents in the Greek mystery religions as well as parallels in Eastern philosophy. It is further developed in the Corpus Hermeticum, in Christian theology, Islamic Mysticism, soteriology and mysticism, and is an important factor in the historical development of monotheism during Late Antiquity.