esoteric

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Intended for or understood by only a small group, especially one with specialized knowledge or interests: synonym: mysterious.
• adjective Relating to or being a small group with specialized knowledge or interests.
• adjective Not known by or suitable for the public; private.
• noun One that is esoteric.
• noun Esoteric matters; esoterica.

from The Century Dictionary.

• Literally, inner: originally applied to certain writings of Aristotle of a scientific, as opposed to a popular, character, and afterward to the secret or acroamatic teachings of Pythagoras; hence, in general, secret; intended to be communicated only to the initiated; profound.
• In embryology, endoblastic. See the extract.
• noun An esoteric doctrine.
• noun A believer in esoteric doctrines.

from the GNU version of the Collaborative International Dictionary of English.

• noun An esoteric doctrine or treatise; esoteric philosophy; esoterics.
• noun One who believes, or is an initiate, in esoteric doctrines or rites.
• adjective Designed for, and understood by, the specially initiated alone; not communicated, or not intelligible, to the general body of followers; private; interior; acroamatic; -- said of discussions of technical topics and of the private and more recondite instructions and doctrines of philosophers. Opposed to exoteric.
• adjective Marked by secrecy or privacy; private; select; confidential.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Understood only by a chosen few or an enlightened inner circle.
• adjective Having to do with concepts that are highly theoretical and without obvious practical application; often with mystical or religious connotations.
• adjective Confidential; private.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective confined to and understandable by only an enlightened inner circle