Esoteric (adjective)

1. Intended for or likely to be understood by only a small number of people with a specialized knowledge or interest.
2. Abstruse, recondite.

Origin:

Late 16th century: from greek esōterikos "belonging to an inner circle", from esō "within" + -teros (comparative suffix).

Examples:

1. He studied an esoteric branch of mathematics.
2. The book is filled with esoteric ideas and concepts.
3. He was an expert in an esoteric field of study.
4. The lecture was too esoteric for most of the audience to understand.
5. Some religious texts are considered esoteric and are not meant for general public.