So goes the traditional, humorous addendum to the celebrated "5, 4, 3, 2, 1" theology student's mnemonic for remembering trinitarian doctrine:
5 notions, 4 relations, 3 persons, 2 processions, 1 substance
So, in all of the obscurity of theological study, one should celebrate a moment when one of the great doctors admits that it ain't so easy to understand this stuff:
"The Greeks also have another word, hypostasis, but they make a distinction that is rather obscure to me between ousia and hypostasis..."
~Augustine, De trinitate, V:10, in Edmund Hill's translation in this edition from New City Press.
Obscure, indeed, not in the least because hypostasis transliterates to substantia, even though we Latins use persona to describe what the Greeks call hypostasis while they use ousia to indicate what we call substantia. That's way beyond terminological drift; it's quite literally confused.