adjective

intended for or likely to be understood by only a small number of people with a specialized knowledge or interest.

esoteric philosophical debates

abstruse,obscure,arcane,recherché,rarefied,recondite,abstract,difficult,hard,puzzling,perplexing,enigmatic,inscrutable,cryptic,Delphic,complex,complicated,involved,over/above one’s head,incomprehensible,opaque,unfathomable,impenetrable,mysterious,occult,little known,hidden,secret,private,mystic,magical,cabbalistic,involuted