De Moor V:1: Trinitarian Terminology, Part 1
Our AUTHOR, being about to treat of the TRINITY of Divine PERSONS, by way of preface takes up a consideration of both terms, Trinity and Person.
Trinitas/Trinity is, as it were, Trium Unitas, Unity of Three, out of 1 John 5:7, so that, although this term is not extant in Sacred Scripture formally and in this composition, it is found there in the passage cited in its basic elements; and so it is not to be rejected as completely ἄγραφος/ unwritten.
The Greeks made use of a less complete term, Τριάδα/Triad, which term according to it origin denotes any number of ternary things; but which, like many other things, was made sacred by Ecclesiastical usage, and was applied κατ᾽ ἐξοχὴν, pre-eminently, in designating the Mystery of the Trinity. But, so that they might further restrict and clarify this more common term, they added epithets, ἁγίας/holy, θείας/divine, προσκυνητῆς, worthy of worship, ὁμοουσίου/ consubstantial, etc.: see SUICER’S Thesaurum ecclesiasticum, on the term Τριάς/Triad.
By the term Triplicitas/Triplicity, the Ancient Dutch were wont to set forth this Mystery, de Heilige Drievuldigheid or Drievoudigheid; but less aptly, on account of the manifest composition, which both the Latin and Dutch terms, derived from tribus/three and plicis/folds, involves, without any indication of Unity. Much more suitable is the vernacular word Drie-eenheid, which we maintain to signify the same thing as the Latin term, Trinitatis/Trinity, intending Trium Unitatem, Unity of the Three. For, although the term Trinitas/Trinity is also able to be used more broadly for a ternary distributive number, commonly it is applied to the Plurality of Persons in the Unity of the Divine Essence.
 John Caspar Suicer (1620-1684) was a Swiss theologian and philologist. He studied at Saumur and Montauban, and served as Professor of Hebrew and Greek at the University of Zurich (1660). His Thesaurus ecclesiasticus was invaluable in the study of the Greek Fathers, shedding light upon words and expressions untreated by lexicographers.