Esoteric
noun
An esoteric doctrine or treatise; esoteric philosophy.
One who believes, or is an initiate, in esoteric doctrines or rites.
adjective
Intended for or likely to be understood by only a small number of people with a specialized knowledge or interest, or an enlightened inner circle.
“The writing in this manual is very esoteric; I’d need a degree in engineering just to understand it!”
Having to do with concepts that are highly theoretical and without obvious practical application; often with mystical or religious connotations.
Confidential; private.
“an esoteric meeting”