Merriam-Webster Collegiate Dictionary - esoteric

 adjective  Etymology: Late Latin ~us, from Greek esoterikos, from esotero, comparative of eiso, eso within, from eis into; akin to Greek en in — more at in  Date: circa 1660  1.  a. designed for or understood by the specially initiated alone a body of ~ legal doctrine — B. N. Cardozo  b. requiring or exhibiting knowledge that is restricted to a small group ~ terminology ~ strategies; broadly difficult to understand ~ subjects  2.  a. limited to a small circle engaging in ~ pursuits  b. private, confidential an ~ purpose  3. of special, rare, or unusual interest ~ building materials  • ~ally adverb