HYPOSTASIS. - It is the transcription of the Greek word ὑπόστασις, which has the same etymology as the Latin words substantia, subsistentia, suppositum, and literally means that which stands under, that which remains beneath the flow of appearances, that which is posited by thought beneath phenomena. It is a very important theological term, whose meaning was made precise in the language of the Church, of the Fathers and of scholastic philosophy as a result of lively controversies.
Apart from the common meaning it had in popular and literary usage, hypostasis, in Greek philosophy and in the Fathers, meant first of all the proper essence of a thing, as a synonym of οὐσία: what a thing really is, its fundamental and specific content: its own nature (οὐσία). Hypostasis was therefore also a synonym of nature, which is conceived as the foundation of all the properties of a thing, as the principle of its specific activities. In St. Paul it means precisely foundation ("hypostasis of things hoped for," Heb. 11:1), principle ("image of his principle," ibid. 1:3) and also essence.
Secondly, in Greek philosophy and in the Fathers, hypostasis had the meaning of "he who has subsistence, or essence or nature," thus becoming a synonym of existing individual or person.
Having to enunciate the notions of the Christian mysteries with terms of common language, the Fathers found themselves faced with possible misunderstandings and were forced to give the terms a more proper and more precise meaning. Thus for the mystery of the Most Holy Trinity the word hypostasis became a source of equivocation, whether by saying that in the Godhead there are three hypostases, because it could be misunderstood in the sense of three essences or substances or natures (Arian heresy); or by saying that in God there is only one hypostasis, because it could be misunderstood in the sense of a nature subsisting as a person (heresy of Sabellius). Whence St. Jerome said that in the word hypostasis poison lay hidden (Epist. XV, to Pope Damasus). The Synod of Alexandria, in 362, left each believer free to profess that in God there is one hypostasis or that there are three hypostases, precisely because of the twofold sense of that word (essence-substance and person). It was not possible, however, to remain in equivocation, and it was the clarifying work of the Fathers, against the ambiguities of the heretics, to attach to the terms a precise technical meaning. Thus hypostasis, being distinguished from οὐσία (οὐσία) and from physis (οὐσία), assumed the proper meaning of an individual subsisting by itself and in itself, that is, of person, and the Trinitarian dogma received its definitive orthodox formulation: one sole οὐσία or physis (= substance or essence or nature) in three hypostases (= three persons).
Thus for the enunciation of the mystery of the Incarnation equivocations arose, if the sense of hypostasis was not well defined. If indeed it was taken in the sense of οὐσία and physis, one had to say, in order to remain within orthodoxy, that in Christ there are two hypostases, because by saying that in Christ there is only one hypostasis one affirmed the fusion of the natures, human and divine, falling into Monophysitism.
If instead it was taken in the sense of person, one had to say, in order to be orthodox, that in Christ there is only one hypostasis, so as not to fall into Nestorianism.
By defining instead the meaning of hypostasis as that of person or being subsisting by itself and in itself, the orthodox formula of the mystery of the Incarnation remained fixed as follows: in Christ there is one sole hypostasis (the divine person, the Word) and two οὐσίαι or physeis (the divine nature and the human nature); the union therefore of the Word with human nature was called hypostatic union (v.).