The term miaphysitism has been used to describe the Chritology of the Syriac Orthodox Church. The best way to define miaphysitism is to quote Mor Severus, the Patriarch of Antioch. He says:
"We must confess the difference between the two natures from which the one Christ is, and avoid the cutting into two, and extol one Son and Christ, and one incarnate nature of God the Word. "
This comes from the following Letter of Mor Severus to Maron.
Enough has, I think, been said about essence (ousia - Gk) and hypostasis (qnuma - Syriac). But the name 'nature' (physis - Greek; kyana - Syriac)is sometimes taken in place of ‘essence', sometimes in place of hypostasis.
For even the whole of mankind we call comprehensively 'nature', as it is indeed written: “For all natures of beasts and of birds, and of reptiles and of things that are in the water are subjected and are made subject to human nature” (James 3:7) and again we speak of one nature in reference to a single man, Paul for example or Peter, or maybe James.
Where therefore we name all mankind one nature, we use the name 'nature' generically in place of 'essence'; but, where we say that there is one nature of Paul, the name 'nature' is employed in place of 'individual hypostasis'. So also we call the Holy Trinity one nature, employing the term 'nature' in place of the general designation 'essence'; as Gregorios the Theologian the bishop of Nazianzus also said in the sermon on the Holy Pentecost:
“Confess the Trinity to be of one Godhead, my friends; or, if you like, of one nature; and we will ask for you from the Spirit the expression 'God” (Gregory the Theologian Oration XLI:8)
But, when we say 'one incarnate nature of God the Word', as Athanasius the prop of the truth and the apostolic faith said in the books on the Incarnation of the Word, we use 'nature' in place of 'individual designation', denoting the one hypostasis of the Word himself, like that of Peter also or of Paul, or of any other single man. Wherefore also, when we say 'one nature which became incarnate', we do not say it absolutely, but by adding 'one nature of the Word himself clearly denote the one hypostasis. But the very men who blasphemously call the one Christ, two natures use the name 'nature' in place of 'individual designation', saying that the ‘Word of God’ is one nature, and the man as they say from Mary another. For they do not reach such a height of fatuity as to say that they are using the name 'natures' in place of 'general designation', I mean in the same sense as essence: for, if the Holy Trinity is one nature, and all mankind one nature, in the same sense as anything which is shown to be so on this principle, the Holy Trinity will be found (to say a very absurd thing) to have become incarnate in all mankind, that is the human race.
But the Holy Scriptures instruct us otherwise, teaching us that God the Word one only of the three hypostases became incarnate and humanized. For 'the Word became flesh, and dwelt in us'.
But, when you hear these things, you will perhaps say that we ought not to have spoken of difference between the natures from which Emmanuel is, lest we ourselves be found to be repeating and using the same expression as these proud men. Accordingly, let us also refrain from confessing the union, because they also profess to speak of a union which consists in an association of honour; and, because they speak of two natures after the union, let us also not say that the union was made from two natures, rejecting even the very mention of natures, like silly children, who tremble at terrifying alarms that are fictitious and invented, as if they were truth, and flee to their mothers' bosoms. If on account of the blasphemies contained in the opinions of those men we yield to them words and names which establish the truth, together with the sound of the words the great mystery of religion goes from us. But, if we be right-minded, we shall both religiously hold to the words and cast out the foul opinions as evil speaking.
You see that we must confess the difference between the natures from which the one Christ is, and avoid the cutting into two, and extol one Son and Christ, and one incarnate nature of God the Word.