‘HOLINESS’—‘the holy’—is a category of interpretation and valuation peculiar to the sphere of religion… it contains a quite specific element or ‘moment’, which sets it apart from ‘the Rational’… and which remains inexpressible—an arreton or ineffabile—in the sense that it completely eludes apprehension in terms of concepts.
Otto introduces ‘numinous’ as the irreducibly non-rational, ineffable core of the holy, establishing the foundational definition upon which all subsequent depth-psychological usage depends.
, The Idea of the Holy: An Inquiry into the Non-Rational Factor in the Idea of the Divine and Its Relation to the Rational, 1917thesis