Adjective: Esoteric (es-uh-TAIR-ik)
1. a: Designed for or understood by the specially initiated alone.
b: Requiring or exhibiting knowledge that is restricted to a small group; difficult to understand.
2. a: Limited to a small circle.
b. Private, confidential.
3. Of special, rare or unusual interest.
Sample sentence: Some of the terms used in the book were so esoteric that I had to look them up in a special glossary that really should have been appended to the book itself.