Mysticism (Oxford English Dictionary):
1. belief that union with or absorption into the Deity or the absolute, or the spiritual apprehension of knowledge inaccessible to the intellect, may be attained through contemplation and self-surrender.
2. belief characterized by self-delusion or dreamy confusion of thought, esp. when based on the assumption of occult qualities or mysterious agencies.
I have long been a bit of a hard nut, not given to believing in 'mysterious agencies,' and indeed I have come actively to disbelieve anything written in a newspaper, and require stringent proofs for any important claims. I am a doubter.
Even dictionaries, of course, can get things wrong. Be that as it may, the Oxford Dictionary gives the Greek root of the word mysticism as muein - to close the lips or eyes. Plato said that we come into this world knowing much, but we forget it all. Someone has put a seal on our lips. At the same time, much that is worth knowing cannot be spoken.
There are, indeed, things that cannot be put into words. They make themselves manifest. They are what is mystical. Wittgenstein, Tractatus 6.522