esoteric (adj.) Look up esoteric at Dictionary.com
1650s, from Greek esoterikos "belonging to an inner circle," from esotero "more within," comparative adverb of eso "within," related to eis "into," en "in" (see en- (2)).

In English, originally of Pythagorean doctrines. According to Lucian, the division of teachings into exoteric and esoteric originated with Aristotle.