I like this compact account of Trinitarian terminology in Vladimir Lossky’s Dogmatic Theology, 38-40. It’s especially good at showing the relation between the words ousia and hypostasis. Watch how Lossky relates the key terms to each other and uses them to draw out how they function in Trinitarian doctrine:
The great problem of the fourth century was to express divine unity and diversity simultaneously, the coincidence in God of the monad and the triad. With the Fathers, one witnesses a true transmutation of language, using either philosophical terms or words of the current language, they transform their meaning until they are able to encompass this prodigiously new reality, which Christianity alone reveals: that of person…”
To express the reality common to the Three… the Fathers singled out the word ousia; it belonged to the philosophical language with meaning of “essence,” though it had become a commonplace to mean, for example, a “characteristic” or a “property.” The word had an ontological resonance, related as it was to the verb eimi (to be), and could well emphasize the ontological unity of the divinity, especially as it is found again in the term homousios, already christianized by the Council of Nicaea.
Homoousios and ousia, however, insisted on identity and there we had a familiar step into the thought of late Hellenism… Homoousios already introduced an immense innovation, since the identity of essence that it expressed united two persons irreducibly different, without absorbing them in their very unity…
Lossky focuses on the theological need to pick out precisely this irreducible difference, and argues that patristic theology had to do this in an intellectual context that had plenty of ways to talk about metaphysical unity, but no ready vocabulary for the kind of equally ultimate personal distinctness revealed in Christ’s relation to the Father:
…for the Latin persona, the Greek prosopon marked only the restrictive, deceptive, and finally illusory aspect of the individual: not the open face of personal being, but the face-mask of impersonal being. Indeed, prosopon is the mask or the role of an actor, the other here is entirely superficial, and as such it has no ontological density whatsoever. Thus, it is not surprising that the Fathers preferred to this weak and perhaps misleading word, a word without a mortgage, whose whole meaning they entirely remodeled: hypostasis.
(The translator has a handy footnote here: “without a mortgage” is a French idiom. We might parallel in U.S. idiom as “with no baggage,” or “no strings attached.”) The point is that prosopon comes with a host of theatrical associations of unreality; hypostasis has fewer associations, and the ones it has are more helpful:
While ousia seems to have been a philosophical word in the act of becoming a commonplace, hypostasis was a current word that had begun to take on a philosophical meaning. In everyday language, it designated subsistence, but with certain Stoics it had taken on the meaning of a distinct substance, of the individual. On the whole, ousia and hypostasis were near synonyms, both relating to being; the first denoting rather the essence, the second the singularity; the divergence, however, should not be pressed too far… This relative equivalency favored the elaboration of a Christian language: no preexisting context would come to dissolve the equilibrium of the two terms whose equal dignity the Fathers intended to emphasize: one avoided the risk of giving more weight to the impersonal essence. Ousia and hypostasis were practically synonyms at the outset, and both were related to the sphere of being; by specializing their meaning, the Fathers were able to implant being in the person, and personalize ontology, without an external obstacle.
The near-synonymousness of ousia and hypostasis is sometimes considered a problem for pro-Nicene theology. Lossky, rightly I think, brings out how it’s actually an advantage that both words are being-words: it’s not because God is simultaneously one being and three beings, as if we were just switching nouns to keep people from noticing the palpable contradiction. No, by pressing the being-word hypostasis into service to pick out the three in God, Nicene Trinitarianism asserted that a person of the Trinity is a way of being God; “person” is at least not un-ontological.
I find this brief discussion in Lossky’s Dogmatic Theology more helpful than his longer (and I admit,very similar) treatments in places like Orthodox Theology (see at p. 40) . It’s more portable, for one thing: you can read this bit of Lossky for its explanatory power regarding Trinitarian terminology, without having to buy into some of Lossky’s programmatic commitments and architectonic accounts of the structure of Eastern and Western thought in antiquity and modernity. In my selective quotations above, I’ve tried to streamline his contribution to explaining terminology even more, in order to interact with Lossky’s helpfulness without simply having to make an issue of his system. Dogmatic Theology is from lectures Lossky gave in Paris in the late 1950s (right up to a time very near his early death). It represents a looser, spoken form of material he comes to us without some of the elaborations he gave these arguments in earlier, published forms.