What is mysticism?

So the beginning of an answer to the question, ‘What is mysticism?’ must be this: mysticism is the passionate longing of the soul for God, the unseen reality, loved, sought, and adored in himself for himself alone.
–Evelyn Underhill (1875-1941)

The mystics are…

The mystics — to give them their short, familiar name — are men and women who insist that they know for certain the presence and activity of that which they call the Love of God. They are conscious of that Fact which is there for all, and which is the true subject-matter of religion; but of which the average person remains either unconscious or faintly and occasionally aware.
–Evelyn Underhill (1875-1941)