Brewer's: Hypostatic Union
The union of two or more persons into one undivided unity, as, for example, the three persons of the eternal Godhead. The Greek hyposiasis corresponds to the Latin persona. The three persons of the God and three hypostases of the Godhead mean one and the same thing.
“We do not find, indeed, that the hypostatic pre-existence of Christ was an article of their creed [i.e. of the Nazarenes].” —