numinous

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'."

Numina was also a Roman term, denoting the "spirits" that their worldview had surrounding them -- Spirits of the trees, water, animals and also abstract concepts like war and drought. According to Ninian Smart (perpetrator of The Seven Dimensions of Religion), Rudolph Otto coined the term to refer to "the feeling aroused by a mysterium tremendum et fascinans, a mysterious something which draws you to it but at the same time brings an awe-permeated fear. It is a good characterization for many religious experiences and visions of God as Other. It captures the impact of the prophetic experiences of Isaiah and Jeremiah, the theophany through which God appeared to Job, the conversion of Paul, the overwhelming vision given to Arjuna in the Hindu Song of the Lord, the Bhagavadgita. At a gentler level it delineates too the spirit of loving devotion, in that the devotee sees God as merciful and loving, yet Other, and to be worshipped and adored."

Standing in contrast to the numinious type of religious experience is the mystical type, where the quest for God is an internal, contemplative one--seeking the Divine Being within. There are also conversion experiences, such as being "born again"; and shamanic experiences, where a person goes on a vision quest to acquire knowledge and power.