Esoteric
/ˌes.əˈter.ɪk/ adjective mediumUnderstood by or intended for only a small group with specialized knowledge.
The seminar covered such esoteric aspects of tax law that even some lawyers in the audience looked lost.
Memory Trick
The word comes from a Greek term meaning 'further within' — esoteric knowledge is kept further inside a small circle, out of reach of outsiders.