Rudolf Otto (1869-1937), in his book The Idea of the Holy (tr. 1926; orig. 1917), defined the Tremendum as an aspect of the numinosum, ‘numinosity,’ which for him was the central characteristic of the Divinity.
Jung’s editorial gloss establishes the precise Ottonian definition: tremendum is a constitutive aspect of the numinosum, designating the overwhelming character of divinity.
, Letters Volume 1: 1906-1950, 1973thesis