For a tight definition, I am not of much help to you here, sorry.
The key goal of the confession really was to establish quite what Christians should not be believing and saying, and flushing out some of the controversial beliefs in the early church, while helping Theodosius, the Roman Emperor, to unite a crumbling Roman empire into a more-defined and less-divided faith.
The focus throughout Church history, however, has been on comprehending what believers are to believe about God, and thus almost every word of that statement has been analysed to unbelievable levels in the attempt to understand what could be meant precisely by this language and its implications for the church and individual followers of Christ.
Surprisingly enough, “trinity” does not originally mean very much other than a threesome, or simply “threeness”. Consequently, when it started being borrowed into theological discussions concerning the Christian God, I contend it did not originally mean the single-being-yet-tripersonal God in the second and third centuries (the period separating the New Testament and the beginning of the creedal period). The Latin word used by Tertullian in the third century, trinitas, was a plural referring term, with God being the founding source member of the other two, who are not properly called God or understood to be God in the same way, but were instead derived or sent out from within God. (Please refer to Chapter 4 for a quick examination of a variety of references on this evolution, including Tertullian). I believe it is important to realise that even the language provided by the fourth and fifth century Catholic Church (trinity as a theological word still being a fairly recent word), already provided a subtle evolution in the term “trinity”, I contend, from its earlier theological use.
As the church entered the Middle Ages, well after the trinity came to be understood as three co-eternal, consubstantial and co-equal persons in one perfect godhead, the Trinity, there was a further significant shift – for some – in trying to figure out exactly how the Holy Spirit is sent. This debate became a source of division and permitted the eastern and western churches to split, the East preferring the name “Orthodox” and sticking resolutely to the original formulation of Constantinople regarding the Holy Spirit. The Catholic Church resolutely defended the addition of the “filioque clause”, which means that – for them – the Holy Spirit proceeds from both the Father and the Son.
Another very significant swing in Trinity theory is the so-called Trinitarian “revival”, which is much more recent, alive today and has roots from within both the Catholic and Orthodox wings, and has been developed differently throughout denominations. The Trinity has become a source of lots of reflection (and speculation) about how God is like a perfect community, or church or even society (we will explore the fact of multiple expressions of what people actually mean by “Trinity” today in the very next chapter).
Stephen Holmes explains how recent developments and theories of the Trinity are in actual fact a departure from divine simplicity and the oneness of God maintained throughout most of church history. Thus the enthusiasm and charisma of many modern expositors such as Ravi Zachariah, Leonardo Boff or G. K. Chesterton, who might delineate through word play the “tri-unity” of God, or expound that “God himself is a society”, can legitimately be described as a subsequent development or even an interpolation of what the word T-R-I-N-I-T-Y has meant historically. So we have a complex matrix of meaning here to untangle. One thing is certain: there is not a single theological meaning commonly understood throughout church history, right up to today, and there exists no one sentence or formulation of ideas that can adequately state them all, and they cannot be mutually compatible.
So historically, there is considerable movement in the notion of what Trinity might actually mean, and it seems likely to continue to shift. But suffice it to say, that the question of “the” Trinity, is a far, far more complex issue than a “do you or don’t you believe it”, precisely because what is understood by “it” varies and has varied so much. This is a key point of this paper. Let me repeat again, any dream of consensus on what the doctrine actually is disintegrates as soon as you attempt to go any further than the traditional language, which is vague, apparently self-contradictory and necessarily invites disagreement.
Another difficulty, then, in providing a precise definition, in addition to the huge variety of interpretations of what the words actually mean, is that it does not seem at first glance to make sense! It is one of the reasons why there are so many views on it.
Take for instance the simple word “is” if you want to affirm that “Jesus is God”. Is this “is” identical to “=“? Or to put it in an even more confusing format, is the “is” identical to “identical to”, and is the “is not” the exact opposite of “is”? That might seem trivial, but ask yourself the question: if God is the Trinity and the Holy Spirit is God, then is the Holy Spirit not then the Trinity? That would seem absurd, right? So the “is” is a problem. But then we get to greater and trickier questions still, of substance, essence, persons and even personality.
Sometimes apparently simple terms become not-so-simple when you look at them in more detail! In this paper, when I refer to “Trinity” or “Trinitarian” with a capital “T”, I am referring quite specifically to an authoritative, ecumenical, fourth-century, full Nicene confession of the One God in three divine co-equal, co-eternal, consubstantial and unconfused Persons or hypostases (and by “full”, we also have to refer to the 381 version of the creed, not simply the 325 version), and I am necessarily not referring to anything precise in terms of its subsequent development, of what on Earth those words might actually mean in the positive sense (remember the key goal of the confession really was to establish quite what Christians should not be believing and saying).