"any mode of thought or life in which reliance is placed upon a spiritual illumination believed to transcend ordinary powers of understanding," 1736, from mystic (adj.) + -ism. Often especially in a religious sense, and since the Enlightenment a term of reproach, implying self-delusion or dreamy confusion of thought.
Mysticism and rationalism represent opposite poles of theology, rationalism regarding the reason as the highest faculty of man and the sole arbiter in all matters of religious doctrine; mysticism, on the other hand, declaring that spiritual truth cannot be apprehended by the logical faculty, nor adequately expressed in terms of the understanding. [Century Dictionary]