Synonyms: abstruse, recondite, deep
Definition: difficult to penetrate; incomprehensible to one of ordinary understanding or knowledge
Usage: the professor's lectures were so abstruse that students tended to avoid them; a deep metaphysical theory; some recondite problem in historiography
Similar words: esoteric
Definition: confined to and understandable by only an enlightened inner circle
Usage: a compilation of esoteric philosophical theories