Most dictionary definitions state that a mystic is “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”.
But definitions are written by those who have never experience union with the source of existence, and so usually do not believe in such a union. But here is a definition by a great intellectual who experienced such a union, “In mystic states we both become one with the Absolute and we become aware of our oneness. This is the everlasting and triumphant mystical tradition, hardly altered by differences of clime or creed. In Hinduism, in Neo-Platonism, in Sufism, in Christian mysticism, in Whitmanism, we find the same recurring note, so that there is about mystical utterances an eternal unanimity which ought to make a critic stop and think, and which bring it about that the mystical classics have, as been said, neither birthday nor native land.” William James