A theophany is a visible manifestation of God to humans. For example, in the Bible in Isaiah 6, the story of Isaiah seeing the Lord sitting upon a throne is called a theophany.
According to the philosopher Philo, God is purely transcendent, so his interactions with the material world are through an expression of himself, his Logos, or Word. It was through this Word that God created the world.
Traditional Christianity posits that God is a trinity; it also teaches that God became especially immanent through the Incarnation of Jesus, who is at once fully God and fully human.
The Feast of Holy Theophany in the Eastern Orthodox Church on January 6, celebrates Jesus' baptism in the Jordan, at which time both Jesus' divinity and the Holy Trinity was made manifest by the appearance of Jesus being baptised by Saint John the Baptist, the Holy Spirit descending upon him in the form of a dove, while the audible voice of the Father ("This is My Son in whom I am pleased") was heard.
In Greece as well as around the world where there are communities of Greek people, Holy Theophany is celebrated by the blessing of the waters. Traditionally, the priest who reads the blessing will then cast a large crucifix into the waters and young men will dive after it to retrieve it.