Something that excites wonder or curiosity. Something mysterious is usually not well known about in part or in whole.

Mys*te"ri*ous (?), a. [F. mysterieux. See 1st Mystery.]

Of or pertaining to mystery; containing a mystery; difficult or impossible to understand; obscure not revealed or explained; enigmatical; incomprehensible.

God at last To Satan, first in sin, his doom applied, Thought in mysterious terms. Milton.

Syn. -- Obscure; secret; occult; dark; mystic; cabalistic; enigmatical; unintelligible; incomprehensible.

© Webster 1913.