Definition of Theosis
literally, the condition or the state of deity; also, the deification of man

"...while God's inner essence is forever beyond our comprehension, His energies, grace, life and power fill the whole universe, and are directly accessible to us...When Orthodox [Christians] speak of the divine energies, they do not mean by this an emanation from God, an intermediary between God and man, or a thing or gift that God bestows. On the contrary, the energies are God Himself in His activity and self-manifestation. When a man knows or participates in the divine energies, he truly knows or particpates in God Himself, so far as this is possible for a created being...we are able to affirm the possibility of a direct or mystical union betweeen man and God - what the Greek Fathers term the theosis of man, his deification...There is union, but not fusion or confusion. Although oned with the divine, man still remains man; he is not swallowed up or annihilated, but between him and God there continues to exist an I-Thou relationship of person to person."

+Bishop Kallistos Ware
from The Orthodox Way (1995-St. Vladimir's Seminary Press, Crestwood, New York)

For more information on the ancient Christian doctrine of theosis, this is one of the better articles available (by Mystical Rose).
"Grace and the Divinity of Humanity"

Theosis Consulting
Spirit Work
Holy Transfiguration