The terms "mystical" and "mysticism" [...] readily lend themselves to misuse insofar as they are applied to everything inward or intuitive, regardless of level; these words denote all inward contact with realities that are directly or indirectly divine, and not just an exclusively mental contact, and it is only natural that they should suggest above all a spirituality of love, for they are European terms, and Europe is Christian. Their association with the idea of the "irrational" is clearly false; spiritual intuition is suprarational, not irrational. In any case it seems to us that the only legitimate meanings one can attribute to the word "mystical" are on the one hand the meaning traditionally given it by theology and on the other hand an extended meaning based on etymological considerations, as we have just pointed out; this usage clearly has nothing to do with malicious intentions or a simple misuse of language. [Schuon 1970]
The term ‘mysticism,’ comes from the Greek μυω, meaning “to conceal.” In the Hellenistic world, ‘mystical’ referred to “secret” religious rituals. In early Christianity the term came to refer to “hidden” allegorical interpretations of Scriptures and to hidden presences, such as that of Jesus at the Eucharist. Only later did the term begin to denote “mystical theology,” which included direct experience of the divine (See Bouyer, 1981). Typically, mystics, theistic or not, see their mystical experience as part of a larger undertaking aimed at human transformation (See, for example, Teresa of Avila, Life, Chapter 19) and not as the terminus of their efforts. [Stanford, 2014]
- 1736, from mystic (adj.) + -ism. mystic (adj.) late 14c., "spiritually allegorical, pertaining to mysteries of faith," from Old French mistique "mysterious, full of mystery" (14c.), or directly from Latin mysticus "mystical, mystic, of secret rites" (source also of Italian mistico, Spanish mistico), from Greek mystikos "secret, mystic, connected with the mysteries," from mystes "one who has been initiated" (see mystery (n.1)). Meaning "pertaining to occult practices or ancient religions" first recorded 1610s.
Other sources: «Mysticism», 2004, Stanford Encyclopedia of Philosophy article.