arcane

adj. secret, hidden; mysterious; occult

Esotericism refers to the doctrines or practices of esoteric knowledge, or otherwise the quality or state of being described as esoteric, or obscure. Esoteric knowledge is that which is specialized or advanced in nature, available only to a narrow circle of "enlightened", "initiated", or highly educated people. Items pertaining to esotericism may be known as esoterica. Some interpretations of esotericism are very broad and include even unconventional and non-scientific belief systems, typically as contrasted with the "scientific" or "traditional religious" beliefs of the society without or "at large". In contrast, exoteric knowledge is knowledge that is well-known or public; or perceived as informally canonic in society at large.
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Adjective
1. requiring secret or mysterious knowledge; "the arcane science of dowsing"
(similar) esoteric

adj. arcane, mysterious, occult

(a.)
Hidden; secret.