Esoteric /es-uh-ter-ik / adj. 

Definition: Esoteric is often used to describe something that is hidden or secret and intended only for those who have been initiated. Something that is profound and known only to a few. It is used to describe something such as a philosophical doctrine that is confidential.

Etymology: The word dates back to the 1650s and is derived from the Latinized form of the Greek word esoterikos, which means “knowledge which belongs to a certain inner circle.”

In a Sentence

The secretive group holds esoteric knowledge, and they protect it by keeping it hidden from those uninitiated into the group. 

The text in the unearthed documents looked simple, but the meaning was esoteric to the scholars because few of them could understand it.

The song’s esoteric lyrics left it wide open to multiple interpretations.

Synonyms 

Arcane, Mystical, Cryptic, Deep, Profound, Private

Antonyms 

Common, Familiar, Banal, Public, Obvious and Well-Known