This is the orthodox term used to describe the relation of the Father’s nature to the Son’s.  It means that the Father and the Son are of the same (homo) nature (ousios), not similar (homoiousios) or different (heteroousios) as seen in John's assertion that the Word (Jesus) is God (John 1:1). 

In the last phrase (“the Word was God”) John puts theos in an emphatic position (it actually come before “Word”) and does not use an article.  Cults such as Jehovah's Witness and Mormonism have concluded that this means John 1:1 should be translated, “... the Word was a God.”  But this idea betrays a either a lack of knowledge of the Greek language or intentional deception.  Colwell's rule¹ describes the use of anarthous, pre-verbal, predicate nominatives and his conclusions suggest that theos in this position is best understood as Godness or divineness.  The New English Translation (NET) takes this into account and translates John 1:1 as “In the beginning was the Word, and the Word was with God, and the Word was fully God.”  Another tranlsation might be “What God was, the Word was”².  Both of these translations emphasize Christ's divinity and that he is of the same nature as the Father.