Synonyms: mysterious, esoteric, arcane, occult, cryptic, enigmatic
These adjectives mean beyond human power to explain or understand. Something mysterious arouses wonder and inquisitiveness: "The sea lies all about us.... In its mysterious past it encompasses all the dim origins of life" (Rachel Carson).
What is esoteric is mysterious because only a select group knows and understands it: a compilation of esoteric philosophical essays. Arcane applies to what is hidden from general knowledge: arcane economic theories. Occult suggests knowledge reputedly gained only by secret, magical, or supernatural means: an occult rite. Cryptic suggests a sometimes deliberately puzzling terseness: His roommate left cryptic messages alluding to his whereabouts. Something enigmatic is mysterious and puzzling: The biography struggles to make sense of the artist's enigmatic life.