Let's start with a question - Why doesn't Webster 1913 list a definition for numinous here?
Answer - because the term wasn't invented until 1917, when it first appeared in Rudolph Otto's Der Heilige, to be translated into English in 1923 as The Idea of the Holy. The term 'numinous' is the foundation of the entire work.
The whole purpose for writing the The Idea of the Holy was to isolate and define the nature of 'the holy' in a Kantian fashion. To do this, Otto wished to segregate the commonplace meaning of 'holy' as morality or goodness, and get to the more sublime aspects of it. That is, the spiritual, otherworldliness aspect of the holy, absent of any ethical resonances. However, this 'otherness' can only be known via the effects its presence has on those who've experienced it.
(i.e. only those who've had contact with 'God' truly know 'God'. Everyone else acts on the belief of God's existence as told by these 'prophets').
Because the sublime holy isn't of this world, IT can never be understood linguistically (hence at all); only the imprints of its presence on the human mind (memory of the divine presence) can be known. It's these imprints or memories of the divine/holy presence that are 'the numinous', or 'numinous experience'.
As Otto puts it -
"For this purpose I adopt the word coined from the Latin numen. Omen has given us 'ominous', and there is no reason why from numen we should not similarly form the word 'numinous'."
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