An Etymological Dictionary of Astronomy and Astrophysics

1a) Involving or characterized by esoteric, otherworldly, or symbolic practices or content, as certain religious ceremonies and art; spiritually significant; ethereal.

1b) Of the nature of or pertaining to mysteries known only to the initiated.

1c) Of occult character, power, or significance.

2a) A person who claims to attain, or believes in the possibility of attaining, insight into mysteries transcending ordinary human knowledge, as by direct communication with the divine or immediate intuition in a state of spiritual ecstasy.

2b) A person initiated into religious mysteries (Dictionary.com).

Etymology (EN): M.E. mystik, from O.Fr. mistique “mysterious, full of mystery,” from L. mysticus, from Gk. mystikos “secret, mystic, connected with the mysteries,” from mystes “an initiate into the mysteries,” + -ikos, → -ic.